World War Two. Above the White Cliffs of Dover, a fighter plane ascends into the sky and executes a series of daring manoeuvres. The pilot, Philip Peel, comes out of a dive at the last possible moment.

Peel's friend, Tony Garthwaite, is in love with Susan, the daughter of aircraft factory owner John Ridgefield. They marry, and Susan takes Tony home to meet her father and brother, Chris. Chris is afraid of his domineering father, and learns to fly to please him, even though he is not good at it and gets airsick. Ridgefield shows Tony a new jet propulsion engine he is testing. Chris makes his first solo flight, but crashes and is killed. Susan blames her father for Chris's death.

After the war, Tony accepts Ridgefield's offer of work as a test pilot, and he and Susan go to live with him. Susan's relationship with her father grows more strained. He sees Tony as the son he has always wanted, and wants him to help his company to beat the Americans and be the first to break the sound barrier.

Susan becomes pregnant. One day, Tony flies them both to Cairo to see his friend Phil and his wife Jess. Phil is no longer flying, but misses it and accepts a job as another test pilot for Ridgefield's firm. The death of an American pilot is announced in the newspapers; it is thought he was trying the break the sound barrier. This does not dissuade Tony from making his own attempt, but Susan tries to change his mind. She confides her fears to Phil and Jess.

On the day of the test flight, Ridgefield's designer, Will Sparks, tells him that he thinks Phil is a better pilot than Tony. Susan goes to the cinema, to escape the sound of the jet engines. Tony begins the test flight. At Mach 0.9 the plane starts buffeting, but Tony is able to recover and takes it back to 40,000 feet to try again. As his speed approaches Mach 1, the plane dives out of control, disintegrates and crashes, leaving a huge crater in the ground. Phil fetches Susan back from the cinema. She goes to see the crash site. Will tells her that he will never design another jet plane. When Susan sees her father in his office, he is examining the flight recorder. When she hears Tony's last words on the tape, she runs from the room and goes to stay with Phil and Jess, where her baby is born, a son - Anthony John.

Ridgefield is knighted. He persuades Will to return and promotes Phil to chief test pilot. He wants him to fly the rebuilt plane, the 'Prometheus'. Susan buys a flat in London, where she intends to live with her baby son. She is appalled by her father's apparent indifference to Tony's death.

Phil thinks he knows the way to conquer the problems at high speed. Remembering how he almost approached the speed of sound on a flight during the war, he believes that the answer is to reverse the controls, pushing forward instead of pulling the plane back at the crucial moment. Ridgefield asks him to go up in 'Prometheus' and reproduce the conditions under which Tony crashed. Will remonstrates with him, and starts drinking again. Susan asks her father to cancel the test, and tells him she is taking her son away to London.

Phil takes the plane up. Ridgefield asks Susan to come and see him at the airfield. He starts asking questions about her son's future schooling, when Phil's flight commentary is heard. Susan starts to leave, but her father begs her not to leave him alone at such a moment. Phil makes two unsuccessful attempts to reach the speed of sound, but when preparing to land, he decides to put his theory into practice and climbs for a third try. This time he succeeds in breaking the sound barrier, survives and lands safely.

Susan, seeing that her father is not as insensitive and ruthless as she thought, is reconciled and agrees to return home with her son.