(engine roaring) (Dan) In spring 1943... (gunfire) ...133 young men set out on the most daring and ingenious bombing raid in history: to destroy the great dams of Germany using bombs that bounce.

Look at the way it's skipping across the surface of the water there.

If successful, they'll deal a hammer blow to the Nazi war effort.

(shouting) They're led by this man, Guy Gibson.

He's only 24, but he's already one of the most famous and highly decorated pilots in the world.

(dramatic music) I'm Dan Snow, and over three programs, I'm charting the nail-biting countdown to the mission.

In less than two months, 617 will be the most famous squadron number in RAF history.

Last time, we revealed how a secret squadron was assembled at breakneck speed.

Their orders: to learn to fly and drop bombs at dangerously low levels.

"If I tell you to fly through hangar doors," Gibson tells them, "then you will do so, even if your wingtips brush either side."

This time, we count down the critical final days to the raid itself.

Even for an experienced bomber pilot, it's terrifying.

As inventor Barnes Wallis struggles to get the bouncing bomb to work... Barnes Wallis groans, "Oh my god."

...the mission is thrown into jeopardy.

♪ The increasingly dangerous training almost ends in disaster.

(exploding) ♪ And as the target's identity is finally revealed, the crews discover the true danger they face.

♪ (engine roaring) This is the story of the single mission that can change the course of the Second World War.

(thrumming) ♪ These are the dambusters.

♪ (moody music) ♪ RAF Scampton, Lincolnshire.

The aircrew of newly formed 617 Squadron are in their first week of training for their secret operation.

The clock is ticking.

♪ Only Squadron Commander Guy Gibson knows the target.

♪ All the crews have been told is that the mission requires crossing hundreds of miles of occupied Europe at night, at extreme low-level, and they'll have to drop bombs over water with pinpoint precision.

♪ Today, 617 are practicing flying at dangerously low levels, far lower than bomber crews ever normally fly.

Throughout the day, the huge Lancaster bombers are roaring down this airstrip and taking to the skies to go flying at treetop height around Britain.

The crews have been ordered to fly at a hundred feet, but many of the young thrill-seeking pilots are going much lower than that.

(spirited music) The squadron Gibson has pulled together aren't just young; they're a mix of classes and nationalities.

♪ Twenty-seven-year-old pilot Melvin "Dinghy" Young rowed for Oxford and got his nickname after miraculously surviving two ditches into the sea.

♪ Twenty-four-year-old Les Munro is a sheep farmer's son from New Zealand.

He's a crack pilot who's just flown underneath another Lancaster which was only a hundred feet from the ground.

♪ But low-level flying isn't the only problem on Gibson's mind today.

The squadron won't just have to fly low; they'll have to drop their bombs from low heights also, and at exactly the right distance from the targets.

In his office, Gibson is trying to work out how on earth they can do it.

(knocking) There's a knock on Gibson's door.

In walks a wing commander he's never met before.

His name is Charles Dann, and he immediately starts telling Gibson why the bomb-aiming equipment on the Lancasters will be hopeless against the dams.

Understandably, Gibson is very sensitive about secrecy, and here's some man he's never met before, revealing that he knows all about the targets of this top-secret mission.

"How the hell do you know all this," Gibson asks him.

Dann calms him down.

He explains that he's a bomb-aiming expert, and he's just been added to the need-to-know list.

He goes on to tell Gibson how he's got a remarkably simple solution to the problem of how to bomb accurately at low level.

(energetic music) Dann says that for the bouncing bombs to work, not only do they need to be dropped from precisely 150 feet, but the planes must also be exactly 1,350 feet from the dam.

And he's come up with a way for the crews to know when they're at precisely the right distance away.

His solution is an astonishingly simple bombsight.

It's basically just four or five bits of wood knocked together in the workshop, held together with bolts here, with wing nuts, and with a nail on the end of either arm.

The plan was for the bomb-aimer to look through here, and when the towers of the dam were aligned to these nails, it was time to drop the bomb because you were the right distance away.

Bingo.

That's the theory, at least.

(engine starting) (dramatic music) That afternoon, Gibson and Dann fly from RAF Scampton to Derwent Reservoir to test out the crude bomb-aiming device.

(engine roaring) ♪ They both know that if the mission is going to succeed, Dann's bomb-aimer has to work.

(birds chirping) (dark music) ♪ With just over six weeks left until the mission, Guy Gibson is on his way to Derwent Reservoir.

He's been given a new bomb-aiming device to test.

It's meant to solve a major problem: telling you when you're exactly the right distance from the dam to drop your bomb so it'll actually work.

♪ Just like Gibson, I'm going to try it out at Derwent Reservoir.

The journey from Scampton to Derwent is about 50 miles.

Like Gibson, I've got my primitive bomb-aiming device.

(intense music) As we crest the hill, we spot the reservoir and dive down towards it.

♪ (pilot) Imagine trying to do this in a Lancaster.

It's hard enough in this airplane.

(Dan) At this height, one mistake by the pilot would be fatal.

♪ (engine roaring) (soft music) It's worked, just like it did for Gibson that day.

Amazingly, he now has a simple bomb-aiming device that gives you your distance.

And that means one crew member's job has just become even more critical.

(Sir Max Hastings) Suddenly, on the dams raid, the bomb-aimers became enormously important people.

And even if the pilots got it absolutely dead right in the approach to the Möhne or the Eder dams or the Sorpe, they'd achieve nothing unless the bomb-aimer pressed his toggle at exactly the right moment.

(Dr. Onyeka Nubia) So, the bomb-aimer had to have nerves of steel.

They had to be patient.

They had to be calm under fire, and they had to also have a certain disregard for their own lives.

617 Squadron's bomb-aimers now have a way of dropping their bombs at exactly the right distance from the dams.

But they still have another massive problem to overcome.

(engine starting) And that's the equally difficult and crucial issue of low-level bombing, especially at night.

The bouncing bombs have to be dropped from precisely 150 feet above the water.

(intense music) This is astonishingly low and dangerous, and Gibson almost killed himself attempting it a few days earlier.

♪ Not only that; the basic instruments in the Lancaster don't give accurate enough height readings.

If a solution can't be found, and soon, Gibson knows he's going to have to pull the plug on the whole project.

(engine roaring) (ominous music) ♪ Two days later, Gibson selects pilot Henry Maudslay to make a critical trip to Farnborough.

(engine puttering) ♪ Twenty-one-year-old Maudslay is a handsome, rich, Old Etonian.

And he's just been promoted to joint second-in-command to Gibson.

♪ But two years flying bomber missions has aged him.

♪ Maudslay and the other pilots need to know when they're flying at exactly 150 feet, but the Lancaster's instruments are simply not accurate enough to tell them.

Maudslay and the other pilots can't fly low-level at night because of this little dial down here, the altimeter.

This tells you how high you're flying, and they are not that accurate.

They can be up to 30 or 40 feet out, which of course, is no problem when you're cruising along at 10,000 feet, but it is a pretty big issue if you're skimming the surface of a reservoir in the middle of the night.

(engine roaring) (Sir Max Hastings) They always knew that getting the height right was absolutely critical, and they did have all sorts of crazy ideas to do with sort of dangling wires out of the plane and so on.

(Victoria) If you dropped the bomb too low, then it wasn't going to have the bouncing effect that it needed.

If you then drop it too high, then, of course, your accuracy can be completely and utterly thrown off.

(piano music) (Dan) Then a scientist at the Ministry of Aircraft Production comes up with a potential solution that could be an ultra-accurate alternative to the altimeter.

Their ingenious plan is to mount two spotlights on either side of the Lancaster at carefully calculated angles, and that meant that as the plane got lower and lower to the ground, it would form a figure of eight at exactly 150 feet off the surface of the water, the altitude they wanted for dropping the bomb.

(clicking, droning) ♪ The theory was that all the crews would now have to do is look out for the spotlights forming a perfect figure of eight on the water below.

♪ (Sir Max Hastings) The boffins, the scientists, came up with this idea of the twin lights, which had been used before by coastal command for night attacks on U-boats.

But it hadn't worked there because the waves on the sea made it too difficult to see the lights.

But they figured, and they were right, that on the lakes, flat calm, that it was going to work.

(soft music) (Dan) Today, Henry Maudslay is in Farnborough to work out with the engineers how on earth to fix two powerful spotlights to the exterior of the Lancasters.

♪ After four days at Farnborough, Maudslay flies back to Scampton in his Lancaster.

He approaches at dusk, at low level, to test the new system.

Gibson waits here on the edge of the airfield for Maudslay's return.

Suddenly, he sees two thin beams of light in the distance, and a Lancaster bomber emerges from the gloom, to roar at low level across this airfield.

From where Gibson is standing, it looks as if the plane turns, it will gouge one of its wingtips into the ground, it's so low.

Even for an experienced bomber pilot, it's terrifying.

♪ But when Maudslay lands, he gives the thumbs up.

♪ (laughing) Over the next few days, the crews practice using the lights.

Within a week, the entire squadron is flying steadily to within two feet of the desired height.

♪ But there's still one major problem, and that's the bouncing bombs themselves.

When they're dropped at the right height and distance, will they actually work?

(plane droning) (dark music) ♪ To find out, Gibson, Barnes Wallis, and various RAF top brass assemble here at Reculver Beach in Kent.

The entire area is sealed off, and armed guards patrol.

The test that's about to take place is as top-secret as it gets.

Wallis has now finished building a full-size bouncing bomb, and they are all here to see if it works.

Two white aiming buoys are bobbing just out there.

Then, an adapted Lancaster carrying a full-sized dummy bouncing bomb roars up the coast flying at low-level.

For Barnes Wallis, this was the moment when he was going to find out if years of obsessive work had paid off.

It must have been incredibly stressful.

♪ The bomb is released faultlessly.

♪ (whooshing) But as it hits the water, the wooden casing shatters and debris is sent in all directions.

The bomb doesn't work.

(whooshing) As the foaming sea calms, there's a long, shocked silence.

It's only broken when Barnes Wallis groans, "Oh, my God."

He looks so distraught.

Gibson thinks he's about to have a fit.

(soft music) Wallis pulls himself together and marches to the shoreline.

(waves whooshing) He rolls up his trousers and wades into the shallows, hunting for fragments of his shattered bomb.

♪ Gibson hears Barnes Wallis talking to himself.

He says, "They said it wouldn't work.

They said it was too big and heavy, but I'll show them.

I'll strengthen it and we'll have more trials this afternoon."

♪ (Dr. Onyeka Nubia) Barnes Wallis, either through doggedness or sheer bloody-mindedness said, "I'm going to prove my doubters wrong."

(waves whooshing) (Dan) The next bomb run is scheduled for later that afternoon.

As Wallis issues instructions for the bomb's casing to be strengthened, Guy Gibson heads to a nearby RAF airfield to kill time before the next training test.

He borrows a small training aircraft, and flies back to Reculver to examine the bombing range from above.

(moody music) But en route, something terrible happens: the single-engine on his plane... (engine sputtering) ...comes to a grinding halt.

Gibson searched desperately for somewhere to make an emergency landing.

But in this part of Kent, all these fields were full of traps and obstacles designed to prevent invading German troops landing by glider.

Gibson is forced down.

His plane strikes an anti-aircraft obstacle and rolls before coming to a halt in the field.

♪ For a moment, all is still.

♪ Then, Gibson emerges from the wreckage completely unscathed.

♪ Even before the dust settles, a local man runs up and starts shouting, "The problem is nowadays they teach young fellows to fly like you far too early."

Little did he know he was talking to one of the most experienced and decorated bomber pilots in the country.

Then again, he did also have a point.

Gibson was only 24.

(dark music) Gibson decides not to go to the remaining tests that afternoon.

And it saves Wallis more embarrassment, because his hurriedly strengthened bomb also fails to work.

♪ Time is running out, and Wallis's bouncing bomb needs a major redesign.

(splashing) ♪ Five days later, Gibson and the RAF's top brass are back at Reculver.

Wallis has reinforced his bomb and they're trying again.

♪ Gibson later described Barnes Wallis at this very moment.

He said Barnes Wallis is standing right on the edge of the water, a nervous energy to him.

Legs apart, chin jutted out, he said there was an air of fearful expectation about everything.

♪ The first test bomb is dropped... (plane droning, bomb splashes) ...hits the water, hardly bounces, then sinks.

Exactly the same happens with the second.

♪ On the third drop, the wooden outer casing yet again disintegrates.

But then something amazing happens.

The cylindrical steel core inside bounces on, skipping across the water.

♪ This gives Barnes Wallis an idea.

Back at his workshop, he sets to work.

And this is what he comes up with: his new design of bouncing bomb, codenamed Upkeep.

♪ This is one of Barnes Wallis's bouncing bombs.

A steel cylinder inside here would have been three tons of high explosive.

Now initially, Barnes Wallis thinks it needs to be a perfect sphere, round ball, for it to bounce like a marble across the surface of the water.

But during testing, when he encases this in this wooden ball, as the wood smashes off, he sees that this continues to skip nicely in a straight line.

So, he decides to dispense with the wood altogether, which means that the Lancaster aircraft will go with this steel cylinder strapped to the underneath.

(tense music) Three days later, they're back again, this time with Wallis's new cylindrical bomb stripped of its outer casing.

Yet again, the great and the good are expectantly looking on.

♪ The cylindrical bomb is dropped.

It falls perfectly... ♪ ...strikes the water... ♪ ...and sinks immediately without a single bounce.

(somber music) When news of the failed tests reaches the commander-in-chief of Bomber Command, Arthur Harris, he's incandescent with rage.

(Sir Max Hastings) Bomber Harris didn't want to do their dams raid.

He'd become already obsessed with his offensive to burn Germany's cities, city by city, to wreck the whole of Germany.

He thought that so-called precision bombing, complete waste of time.

And all Harris's memos, right through the whole training period, he was always scribbling his intemperate comments on this, saying, "This is tripe of the wildest kind.

It's never going to work.

This is crazy."

He was not a nice man, Harris, but you need people like that in wars.

(dark music) (Dan) On one report Harris scrawls, "As I always thought, the weapon is barmy.

I will not have aircraft flying about with spotlights on in defended areas.

Get some of these lunatics controlled and if possible, locked up."

♪ The raid has to take place in less than a month.

If Wallis can't get his giant bomb bouncing properly, the whole thing will have to be cancelled.

(moody music) Spring 1943.

The water in the great German reservoirs is rising, fast approaching the levels needed for Wallis's bomb to be effective.

If the raid is delayed, the water will start falling.

But that's not the only problem.

Wallis still hasn't got his bomb to work at all.

As Guy Gibson and the crews of 617 Squadron wait for a working bouncing bomb, they practice dropping non-bouncing bombs, at night, on low-level runs here at Derwent Reservoir.

It's a good substitute for the Möhne and Eder dams.

♪ At night, the aircraft come flying down this valley here, only 150 feet above the surface of the reservoir.

Two hundred miles an hour, racing along.

And in front of the dam here there's a raft, about a quarter of a mile out, and they have to drop their bombs as close as possible to that raft.

On the bank, there are gentlemen from the Air Force seeing how close they manage to drop their bombs to that raft.

The crews often manage to get their bombs within 20 feet of the raft.

But things are about to get a whole lot more difficult.

(dark music) ♪ With little over three weeks to go, Gibson gets a call.

Barnes Wallis wants to meet him at his workshop at the Brooklands Aircraft Factory.

He's got something important to discuss.

♪ When Gibson arrives, Wallis tells him he's finally worked out how to make his bombs bounce.

(whirring) But there's a hitch.

It's going to be even more dangerous for the crews than they'd already anticipated, and he wants to ask Gibson face to face if they can do it.

♪ Wallis shows Gibson some calculations he's been working on.

He's come to the shocking conclusion that for this bomb to bounce properly, to work, it has to be dropped not from a 150 feet as they'd hoped, but from 60 feet.

He asked Gibson, "Can you fly 60 feet from the water?"

Gibson thinks about it.

He knows that that's incredibly low.

One false move and they're straight into the drink.

♪ Then he replies coolly, "We'll have a crack at it tonight."

(planes droning) (ominous music) (Mike) As an aerobatic pilot, I've done some flying at low level, but what I haven't done is flown 60 feet over water at night.

And that is just incredible that these guys were expected to go and do that under enemy fire.

And if Gibson had said, "I'm terribly sorry, sir, but it can't be done," that would have been a sensible, rational response.

Gibson was probably one of the few men in the Royal Air Force who was willing to say, "Yes, sir, I think we can do it."

(Dan) Gibson heads back to base to break the news to his men.

The Lancaster spotlights are adjusted from 150 feet to converge at just 60.

Later that night, twenty-two-year-old Dave Maltby and his crew are sent off to have a go.

♪ Maltby, the son of a prep school headmaster, returns saying that 60 feet is very low.

(plane droning) ♪ Then Gibson takes off to try for himself.

(plane droning) He concludes that the spotlight system works well.

♪ Now the rest of the crews must learn how to drop bombs at the new ultra-low height.

(droning) But time is running out.

(dark music) ♪ With less than three weeks until the raid has to take place, Wallis may have got his bomb to bounce on paper, but he still hasn't proved it for real.

He's got one last chance to show his bombs will work if dropped from just 60 feet.

It's 9:15 in the morning, and Gibson and Barnes Wallis are back here on Reculver Beach, waiting nervously for the make-or-break test of the modified cylindrical bouncing bomb.

♪ The Lancaster hurtles in, skimming the sea at 60 feet.

♪ The bomb is released.

(engine roaring) (thudding, splashing) It bounces once... (thudding) ...then twice.

In total, it bounces six times.

It doesn't travel quite straight, a concern Wallis had about a cylinder rather than a sphere, but it skips across the water for more than 2,000 feet before finally sinking.

(uplifting music) Barnes Wallis was euphoric.

The usually calm, buttoned-up, controlled engineer completely let himself go.

He started dancing around, waved his hands in the air.

Then he took out his handkerchief and started waving it above his head.

Gibson couldn't believe his eyes, but he decided to join the celebrations.

He threw his hat in the air and started dancing, too.

♪ Wallis had proved his doubters wrong.

At last, he had his bouncing bomb.

(somber music) ♪ On May the 2nd, after five weeks of intense and dangerous training, the strain is showing on the aircrew.

Gibson decides to give everyone two or three days' leave.

This is completely out of character, because to this point he's been an authoritarian tyrant when it comes to secrecy.

♪ (Sir Max Hastings) It didn't make him loved, but by gosh, they were frightened of his rage.

And he was right to be so strung up, to be so angry, because he knew that their only chance of pulling off this almost impossible feat was if the Germans did not know they were coming.

(Victoria) If the Germans got wind of what was coming for them, then, of course, they could have rearranged and bolstered their defenses.

(Dan) Exhausted and under immense stress, unlike his men, Gibson doesn't take leave.

An angry boil breaks out on his face, and when the doctor recommends two weeks' rest, Gibson laughs.

That's exactly how long he's got before the most dangerous mission of his life has to go ahead.

♪ (Victoria) He was struggling from a variety of different ailments.

So, he had hereditary gout, he was struggling with arthritis in his feet, from the demands of being a pilot of these heavy bombers.

Truth be told, Gibson really shouldn't have been flying.

(Dan) Gibson is so tough and demanding, it isn't easy for his men to like him, but he always leads from the front, and they respect that.

(Sir Max Hastings) Gibson never worried about people's feelings.

He was concerned with one thing: getting the job done.

And whatever it took to do that, he was going to do.

A lot of the men who served under him didn't like him.

Very tough, very unforgiving to weaker vessels than himself.

But if you're going to fight a war, if you're going to wage a bomber offensive, you need a few people like Guy Gibson to do the business for you.

(tense music) ♪ (Dan) Six days before the raid, the first live Upkeep bouncing bombs arrive at Scampton.

The squadron is also supplied with concrete-filled practice bombs.

♪ At last, the crews now start using these for test runs off the coast of Kent.

Dropping even these non-exploding bombs from 60 feet leads to some very close scrapes.

♪ (Sir Max Hastings) Henry Maudslay, a very good pilot, a very brave young man.

And he made a slight mistake and allowed the plane to get a bit too close to the water.

When he drops his bomb, this great gusher flies up.

(splashing) (Dr. Robert Owen) It actually envelops the whole of the tail of the aircraft.

It was amazing, actually, that the aircraft managed to fly through it.

It could quite easily have brought the aircraft down.

(droning) (Mike) I couldn't tell you the levels of respect I have for the 617 Squadron crews that went out to learn this new skill and put it together in such a short period of time.

To do it at night and to the level of accuracy required was incredible.

(melancholy music) (Dan) Returning from a practice bomb run, pilot Dave Maltby takes the opportunity to fly low over the house where his heavily pregnant wife Nina is living.

♪ For security, 617 Squadron is now confined to camp, so this is as close as the married couple can get.

♪ As he swoops low over the house, waggling his wings, Nina takes this photo.

(clicking) She knows her husband is about to take part in a dangerous mission.

This may be the last time she sees him.

(plane droning) ♪ (droning continues) (ominous music) ♪ Gibson is confident his crews are ready, even though only half of them have dropped practice bouncing bombs.

♪ The weather reports also look favorable, with cloudless, moonlit skies forecast over Germany.

♪ But unbeknownst to Gibson, something is happening behind the scenes.

The very top brass in Washington and London are arguing over whether the raid should go ahead or be delayed.

♪ The fate of the operation hangs in the balance.

As a final training exercise, most of the squadron carry out a full dress rehearsal that night.

They follow a route that simulates the trip to Germany and the bombing run against the great dams.

(uplifting music) Twenty-one-year-old intelligence officer Fay Gillon is invited to join the crew of Mick Martin on the dress rehearsal.

Gibson had started inviting ground crew on flights to supposedly "enhance their appreciation of the aircrew's work."

♪ It's possibly only a coincidence that many invited flying are young and female.

♪ Gillon is invited into the engineer's seat as they rush out over the sea and drop down practically into the water.

Her face is pressed to the window as they then swoop back inland, over roofs and houses and around chimneys and church steeples.

♪ When they land, she heads straight to the mess for a beer with the crew.

Then Gibson arrives with a beaming face, saying, "Bloody good show, boys!"

Gibson writes in his logbook, "Full dress rehearsal completely successful."

♪ (dark music) The next morning, an encrypted message is received from the Air Ministry in London, marked "Most Immediate, Most Secret."

It reads, "Operation Chastise approved.

Execute at first suitable opportunity."

Gibson is informed that the raid will take place on the following night.

(suspenseful music) ♪ 4:00 p.m., the day before the raid.

Barnes Wallis flies to RAF Scampton to oversee the loading of the bouncing bombs that for years have been his obsession.

He and Gibson wander around this hangar, watching the ground crews arm and carry out final preparations on each of the 19 aircraft that will be heading to Germany in a few hours' time.

♪ There should be 20 aircraft on the raid, but Maudslay's plane was so badly damaged during the recent training accident it can't be repaired in time.

♪ For security reasons, the ground crews working in here have been told they're preparing for another training mission.

Apart from Gibson, even the aircrews don't know that their actual mission is now just hours away.

But everybody has a sense that this is the real thing.

(dark music) The senior pilots are told first.

At 6:00 p.m., Young, the oldest at 27, and Maudslay and Hopgood, both just 21, are called here to the station commander's house.

Gibson and Wallis reveal the targets.

The meeting continues until midnight.

As they're leaving to go to bed, the station commander pulls Gibson aside and gently breaks some terrible news.

Gibson's beloved Labrador has been run over and killed on a road just next to the base.

Gibson is obsessed with that dog.

It's been his constant friend and companion for two years.

It's traveled everywhere with him.

And now he's lost him the night before the most difficult and dangerous raid of his career.

Barnes Wallis, not usually a superstitious man, thinks this could be a bad omen and worries what impact it may have on Gibson.

(somber music) Gibson himself puts on a brave face, but later admits to sitting alone in his room into the early hours, feeling very depressed.

(dramatic music) ♪ The morning of the raid, and Gibson is up at 5:30 a.m.

He's only had a few hours' sleep.

He's suffering from painful gout in his feet and makes his way to the station doctor before breakfast.

The other aircrew sleep in, then head to the hangar to inspect their aircraft, each one loaded with a single live bouncing bomb.

♪ Mick Martin and his crew clamber on board to inspect their aircraft, codenamed P for Popsie.

Somehow, someone inadvertently presses the bomb release button.

The calipers holding on to the bomb, three ton of high explosives, suddenly snap back, and the bomb crashes onto this floor of the hangar.

People run for their lives.

Fortunately, the bomb doesn't detonate.

If it had blown up, it would have obliterated this hangar and many of the crews of 617 Squadron.

(soft music) ♪ Just before 6:00 p.m., tannoys around RAF Scampton call the aircrews to their pre-mission briefing.

The 133 men selected to fly that night assemble.

♪ They're about to finally hear details of their mission.

♪ Guy Gibson and Barnes Wallis stand at the front of the room.

It's the first time Wallis has seen the faces of the young men, almost all in their early twenties, who'll be risking their lives to carry out his plan.

He turns to Gibson and says, "I hope they all come back."

Gibson, who knows the reality of the odds that they face, replies, "It won't be your fault if they don't."

(Sir Max Hastings) Barnes Wallis was strung out like a piano wire, and how could he not be?

And now, suddenly, he sees 150-odd very young men who are going to execute his idea at the risk of their lives.

Until that point in time, he had always been in control.

Upkeep was his baby.

He had developed it, and I think that that must have been quite a moment for him.

(Dr. Onyeka Nubia) He must have realized at that moment that if his science doesn't work, these young men are going to die.

They're going to die in a failed mission.

And the failure of the mission is going to be his fault.

(dramatic music) ♪ (Sir Max Hastings) In war, the price of failure is paid with lives... ♪ ...and he knew how much was resting on this.

♪ (Dan) When the target is finally revealed, it's actually a relief for most.

They aren't being asked to attack the battleship Tirpitz after all, or the heavily defended U-boat pens in Brittany as they'd feared.

The feeling is at least they have a sporting chance.

(soft horn music) As the men crowd around maps and models of the Möhne, Eder, and Sorpe dams, their primary targets, they're told the locations of known flak anti-aircraft guns on the long, low-level flight across Germany.

♪ Then, Gibson briefs them on the plan of attack.

There are 19 aircraft on the raid.

They will be divided into three different waves.

The first wave, led by Guy Gibson himself, will take this southerly route here to attack the Möhne Dam.

The second wave of five aircraft, led by American Joe McCarthy, will take a northerly but shorter route across the coast first, and attack the Sorpe Dam.

After those aircraft of the first two waves have destroyed their targets, hopefully, they will move onto a third dam, the Eder.

In this, they will be helped by the third and final wave, the reserves, five aircraft who will arrive to finish off any targets not destroyed yet by the first two waves.

(sizzling) The men are given the traditional pre-operation meal of bacon and eggs, a luxury in wartime Britain.

♪ Then, most of the men return to their quarters to write letters home, in case they don't make it.

One of them, who was a dambuster wireless operator, Andy Stone-- and he wrote for his parents in Winchester that if he died over Germany, "I will have ended happily.

So, have no fears of how I ended as I have the finest crowd of fellows in the world with me.

And if the skipper goes, I'll be glad to go with him.

He has so much to lose."

'Cause his pilot's wife was pregnant.

(dark music) We're all tempted to sort of snigger a bit these days at the word "duty," and yet duty was something very real to that generation.

And one of the things that's so moving, both about the dambusters operation and about the whole bomber command story, here were all these young men who knew they were likely to die and, yet, their sense of duty to their mates and the crew, to the squadron, to the RAF, and to their country.

♪ (Dan) The men collect their flying kit and parachutes and begin assembling outside the hangar.

They lounge on the grass in the warm evening sunshine.

♪ Gibson arrives here at the hangar with his entire aircrew packed into his car.

He's exuding an air of relaxed calm, but inside, he's as nervous as everybody else.

Somebody asks, "Do you need anything," and he simply replies, "Just a lot of beer for when we get back," and adds grimly, "hopefully."

Pilots Dave Shannon and Hoppy Hopgood stand behind the hangar smoking and quietly chatting.

Hoppy is one of the most experienced veterans in the squadron and has flown dozens of dangerous missions, but he has a sense of foreboding about this one.

"I think this is going to be tough one," he confides to Shannon, "and I don't think I'm coming back."

♪ (ominous music) Half an hour later, the crews pile into buses that take them to their waiting Lancaster bombers.

♪ A Canadian wireless operator, satirizing Churchill's famous speech, chalks on his bomb, "Never has so much been expected of so few."

♪ And Gibson and his crew climb into G for George.

♪ At precisely 9:00 p.m., a red flare shoots into the dusk sky.

It's the signal for these aircrafts to fire up their engines.

We can imagine what was going through the heads of these men as the huge Rolls-Royce Merlins roared into life.

(engine roaring) They'd been preparing for this for months, but you can never fully anticipate what the enemy is going to throw at you.

(dramatic music) Next time, we follow the daring mission hour by hour, minute by minute.

As they watch, the ground crew hold their breath.

As soon as they cross the enemy coast, the squadron suffers terrible losses... Munro looks to the right, and he sees a burst of anti-aircraft flak followed by a huge explosion.

...but continue fighting their way low-level across hundreds of miles of occupied Europe to Germany itself... Burcher hears the flight engineer shout, "Christ, look at all that blood!"

...and begin their attack on the greatest dams in Europe.

"Bomb is gone!"

♪