Based on a true story, an inventor figures out how to bomb dams in Germany from low-flying aircraft, and a British Air Force squadron trains and carries out the mission.
In spring 1942 a doctor (Charles Carson) visits the home of Dr. B. N. Wallis (Michael Redgrave), who is experimenting with help from his children. His wife (Ursula Jeans) tells the doctor she is concerned her husband is overworking. They hear planes and close curtains for the blackout. Dr. Wallis explains that the Germans need a hundred tons of water to make one ton of steel. He says he is working on a plan to blow up the Ruhr Dam by dropping bombs from the air. The doctor advises him to get some rest.
Dr. Wallis asks a member of the committee to persuade them not to cancel his project. Wallis says he is working on a new plan, but he can’t explain it yet. The committee gives him two more weeks.
Wallis explains to men his experiment with a model. He shows that water between the explosion and the wall absorbs the shock. Then he shows how a smaller bomb explodes the wall. He asks them for a large testing tank to continue his experiments.
Five months later Wallis is still experimenting with the large tank. He gets a golf ball to bounce on the water and hit the wall. The committee comes in, and he repeats his success twice. The committee says it will go to the Ministry of Aircraft Productions. The M.A.P. official (Hugh Manning) tells Wallis that they do not have any extra skilled labor to give him for a revolutionary idea. Wallis says they might approve if they know it is him.
Wallis in an airplane drops a round ball that bounces on the water. He says it works. He goes back to M.A.P. and asks to be able to test the real thing. Wallis tells Captain “Mutt” (Patrick Barr) he is getting the run-around at Whitehall. He asks Mutt to take it directly to bomber command. They go to Air Chief Marshal Arthur Harris, who has read the report. Wallis says he has films to show his experiment works.
At home Wallis tells his wife that Harris is interested but that it is all over. He says Whitehall decided the whole project is a waste of time, and so he resigned. She says the M.A.P. wants to see him in the morning. Wallis says it is all off, but the official tells him that the Prime Minister has approved it. They have only two months to prepare the bomb and train the team.
The chief officer tells Air Vice-Marshal Ralph Cochrane that he wants Wing Commander Guy Gibson (Richard Todd) to be in charge. Gibson lands a plane and tells his dog they are on vacation. Gibson meets with Cochrane, who says it will be low flying at night, and he assigns Captain Whitworth (Derek Farr) to help him select a squadron. Two of Gibson’s men tell him he will need them, and he says all right. Young is second in command.
Gibson tells his men that discipline and security are important, and they will be flying low at night over lakes. Three planes practice. Gibson meets Wallis, who says he has a special type of bomb, and he shows him films of the bomb being dropped and bouncing on the water. An earlier experiment shows how the bomb hits the wall and slides down to be exploded. Gibson asks if a 5-ton bomb will bounce on the water.
Cochrane shows Gibson that the target is the dam. He says they have only one chance that year on a full moon in five weeks. Gibson takes Bob Hay to watch an experiment with a full-size bomb. They watch another test, and the bomb breaks again. Wallis says he has to strengthen the casing. Gibson says accuracy is difficult at night. Wallis wades in the water to find fragments.
Gibson watches a show and notices the spotlights. Gibson flies a plane with two spotlights that help them find the proper level. They watch the experiment again, and the bomb breaks apart again. Wallis says he will try a new method of release and a stronger casing. He asks Gibson if he could fly in at 60 feet. Gibson tries it the next day, but it is more dangerous. A poultry farmer is writing a letter of protest.
At the test Gibson tells Wallis they can fly at 60 feet. This time the bomb bounces on the water, and they are happy. Wallis tells the committee that he got the idea from Nelson who bounced cannon balls into ships.
Cochrane gives Gibson a device to tell when they are six hundred yards away, and in the test the bombs land near the mark. Gibson tells his crews they have dropped 2,000 bombs in practice. The new planes are arriving. They will go in a week. They say they have been ribbed because they have been waiting so long. Gibson approves their fighting back, and they have a brawl. Gibson is told they will do it tomorrow night.
Gibson tells the squadron their training is over, and they are going to attack the great dams of western Germany. He will lead the first planes, which will be followed by two more waves with five planes in each. To avoid fighters they must fly at zero feet going and coming back. Gibson answers some questions. They study models of the dams and learn code words. Outside Gibson is told that his dog was killed by a car. At dinner Gibson says they need different oil in the planes. Wallis comes in and says they got the oil. On the airfield Gibson asks a sergeant to bury his dog at midnight.
Crews wait on the airfield. They get in trucks and are driven to the planes. They board and start their engines. Gibson takes off first along with two other planes. They fly low over the ocean. Wallis waits anxiously. The planes fly over the Dutch coast. Gibson follows a canal. They fly through anti-aircraft fire, and one plane is hit and crashes. Gibson reaches the first dam and faces anti-aircraft fire. Gibson flies in to attack. They use the beacon lights. At the right moment they drop the bomb that skips along and explodes by the wall, but the dam holds. The next plane goes in and drops the bomb that explodes; but the plane was hit and catches fire and explodes. Another plane goes in, and the bomb explodes by the wall. Three more planes fly in, and one drops a bomb. The next bomb breaks the dam, and the water pours out. Headquarters gets the report, and they congratulate Wallis.
Gibson goes on to the next dam. The first plane goes in, and there is no flak; but they are too late and pull out. A second plane goes in, and Gibson sees an explosion on the ground. The plane with the last bomb goes in, and the bomb breaks the dam. The water released causes flood damage. The planes return, and the men drink coffee. Radio reports that two dams were breached, containing most of the water in the Ruhr Valley. Eight planes are missing.
Gibson tells Wallis they know four planes were shot down. He says the flak was worse than they expected. Wallis says if he had known that 56 men would be killed, he would not have done it; but Gibson says they were willing to sacrifice themselves. He says he has to write some letters.
This war drama depicts the detailed and technical preparation used in modern warfare. The targets also reflect the industrial nature of the war that destroys the infrastructure for manufacturing weapons.