Spring 1942. A doctor arrives at the Barnes Wallis home to find Barnes Wallis firing marbles across a bucket of water in the garden. His wife expresses her concerns about her husband's unending work towards the war effort on top of his work at the armaments company, Vickers. Barnes Wallis explains to the doctor his concept of an 'earthquake bomb' for destroying dams.

Barnes Wallis travels to London and persuades the Ministry to give him more time to perfect his new weapon, and with their agreement begins successful trials on model dams at Harmonsworth. This, in turn, leads to further experimentation, firing golf balls across Ministry of Shipping testing tanks at Teddington.

Despite the success of his experiments, his meeting with an official from the Ministry of Aircraft Production suggests shortages will prevent its production: the RAF are unwilling even to arrange the loan of a Wellington bomber for the trials. Barnes Wallis notes he designed it, suggesting that "perhaps that might help?".

Coastal trials of a prototype bomb follow, with Barnes Wallis acting as bombardier. Again, success runs up against Ministry bureaucracy, and Barnes Wallis is directed to Air Chief Marshall Harris at Bomber Command, who agrees to see the films of the trials.

Back at his cottage, Barnes Wallis reveals to his wife the rejection of his ideas and his resignation from Vickers. Called to the Ministry the following day he is told that Downing Street has decided to agree to his plans: he has two months to prepare for a May attack.

Air Chief Marshall Harris forms 617 Squadron from experienced flyers to be hand picked by Wing Commander Guy Gibson. His own crew volunteers to go with him and slowly the Squadron is assembled and commences low level flying practice over British lakes. Gibson is briefed on the precision bombing required, and the secrecy surrounding the new bomb and the mission.

Meanwhile Barnes Wallis is having problems with the bomb trials at Reculver in Kent, as the bombs disintegrate on impact. The answer is to fly lower, but accuracy is a problem. Inspired by a theatre visit, Gibson fits two spotlights, set to line up at a specific height, to the underside of the planes. Further unsuccessful trials result in the aircraft having to fly even lower, though a new bombsight is successfully trailed, overcoming the last obstacle to the mission.

As the Squadron is briefed, Gibson's dog is run over and killed - a bad omen. Gibson requests its burial outside his office at midnight (the time when he should be reaching the target). The flight crews prepare for the mission: writing letters, setting clocks, shaving, and eating.

There is a lull as they wait around the airfield in their flight uniforms. Suddenly a flurry of activity sees them delivered to their aircraft and taxiing for take-off. Barnes Wallis watches them depart.

The bombers fly low over the Channel and across the Dutch coast, attracting enemy fire and losing one aircraft. Gibson's flight circles the Murnur Dam and comes under fire as he goes into his bombing run. Bomb after bomb hits the dam to no avail, with another aircraft lost in a blast. With bombs running low, Gibson brings his bomber in to cover another's run, and the dam is successfully breached.

Back in the Operations Room, there is elation at the news, and Air Marshall Harris rushes to congratulate Barnes Wallis.

Gibson's flight moves on to their next target, the Aida dam, and encounters a difficult approach. The first aircraft gets caught in a blast and crashes; the next delivers its bomb to no avail. Everything rests on the final bomb, which hits the target, destroying the dam. As Gibson's flight heads back to England, water floods from the ruined dams, destroying factories, rail yards and armaments, and flooding vast tracts of land.

Returned to England, the surviving crews return to their rooms, as a radio announcement confirms the raid's success. Gibson and Barnes Wallis discuss this and concern is voiced over the considerable human cost of the mission, with eight out of 14 aircraft missing. Gibson walks into the distance, with a number of letters to write before turning in.