Barnes Wallis (Michael Redgrave
) and his children are apparently playing ducks and drakes (skipping flat stones on water) with the aid of a brim-full tin bath. Subsequent events show that this is far from the case.
Wallis, a dedicated scientist, devotes all the time he has (including family time) to develop a totally new 5 ton bomb. His initial struggles with an uninterested war ministry are graphically portrayed. When he asks for a Wellington bomber to test his bomb, the irate minister says "They are worth their weight in gold. What possible justification can I give for such a request?" Barnes replies, "You could tell them I designed it."
At the Scampton RAF base, a weary Wing Commander Guy Gibson (Richard Todd
) and his crew are completing a tour of 30 sorties over Germany. Just as they are looking forward to well earned leave, Gibson is asked to form a new squadron. By now Barnes is dropping nearly full-sized bombs off Reculver, but they are bursting rather than bouncing over the sea. Again and again, Gibson travels down to the Kent coast only to see another failure.
Wallis asks if the bombers can fly at just 60 feet rather than the 150 feet originally requested. Guy points out that the altimeters are not accurate enough to fly at such low level without crashing. But later Gibson is watching a show in London and the spotlights focusing on the star on stage give him the answer. Lights are mounted in the nose and tail of each Lancaster bomber and set to converge on the ground at 60 feet. The raid is now on but while the crews of 617 Squadron are being briefed, Gibson's faithful dog Nigger is run over. Not a good omen for the coming attack.
Poignant scenes of the very young-looking bomber crews sitting around waiting to depart, culminating in Gibson saying "My watch says it's time to go." Dramatic scenes of crews driving out to their dispersed Lancasters and then engines starting and serried ranks of the huge four-engine bombers lining up to take off. The low-level flight to the target dams commences with some beautiful shots of the Lancs skimming over the moonlit English Channel. Soon enough they hear "Enemy coast ahead," and with a flurry of flak the bombers are twisting and turning their way to their target. Meanwhile Barnes Wallis is in the ops room at Scampton, monitoring the progress of the raid. All is silent and all eyes are on a special phone that will bring news of the raid's progress. 617 Squadron are now well on their way but suddenly get caught by AA guns over a city. One Lancaster is shot down, exploding as it hits the ground.
Finally the planes arrive over the Mohne lake, looking very peaceful in the moonlight. The bombers can only attack one at a time so Gibson makes the first run. He levels out. The bomb aimer raises his simple wooden "coat hanger" bombsight while the navigator watches the lights coming together on the lake surface as the bomber approaches 60 feet. "Bomb gone!" Then a howl from Merlin engines being thrashed at full power to get away from the explosion. For such an old film the dam bomb bursts are well done. The Lancs take turns making their runs. One fails to pull up and its bomb goes over the dam wall, exploding under the plane's tail. Each run is reported back to Scampton and the small group of officers waiting around the silent telephone. Suddenly it rings and morse code can be heard. But time and time again the code word for success is not transmitted.
At the dam the flak has become murderous, so Gibson and another pilot fly their Lancasters in formation with a plane making its bomb run. The escorting Lancs flick their landing lights on and off to draw the guns from the attacking bomber, which releases its bomb, destroying the Mohne dam. The remaining bombers fly on to the Eder dam, where their success is repeated, and back at base the group waiting around the phone celebrates wildly as the news of the raid's success comes through the morse-beeping phone. The film closes with the surviving bombers wearily landing and the tired crews flopping out. Guy Gibson runs into Wallis, who is distraught over the numbers of bombers lost. Guy tells him to turn in and rest. However, before Guy can do so, he "has some letters to write."