1939. The German refugee physicist Crystof Wolters and his Norwegian friend Hjoerdis Lundborg, for whom he cares deeply, meet for lunch in a London park. Wolters has fled the Nazis and is enthusiastic for British life. When Hjoerdis leaves, he is joined by Buschmann, another German. Buschmann soon reveals that he is a communist and he knows of Wolters. Wolters professes no interest in politics, having left the communist party years ago. Despite this, Wolters agrees to meet Buschmann again.

Soon, Wolters is interned in Canada. On the voyage, he shares a cabin with Buschmann and Friedlander, an elderly Jew who had previously spent five years in Dachau. Wolters has become disillusioned with the British. Friedlander dies as a result of his diabetes, the ship having been insufficiently supplied with insulin for its treatment.

1941. Wolters has been allowed to return to Britain, where his expertise is required for the war effort. He is called to an interview with Professor Kettering at a secret establishment. Kettering tells him that their aim is the complete destruction of Germany and that Wolters must become a British citizen if he is to be part of it. Later, Wolters is shocked to discover that Kettering's new wife is Hjoerdis. He is disappointed that she married in his absence and is initially unwilling to work with her.

1943. Wolters is passing reports on his work to Buschmann. Buschmann reveals that he is travelling to Russia to become a Party propaganda officer and that Wolters will have a new contact.

1945. Wolters is now working in Las Mesas, New Mexico, on the American research project which will culminate in the atomic bomb. Wolters is shocked when a colleague reveals that the atomic bomb has been created and the problem of detonation solved, despite Germany's unconditional surrender.

After playing tennis with her, Wolters declares his love for Hjoerdis. She says that her marriage may soon come to an end, hinting that there may be a future for the two of them together. Then she discovers a note detailing Wolters' next meeting with his Russian contact, but she assumes it is for a liaison with a woman and they fall out.

At a party that night Hjoerdis agrees to stay with Kettering out of their mutual need for each other. The staff engage in a heated debate over the morality of using the atomic bomb, with Kettering vigorously advocating the destruction of Japan and Russia. When he receives little support, he suggests treason is afoot, provoking a debate on the nature of treachery. Later, the radio reports the bombing of Hiroshima.

1946. Back in England, Wolters meets his Communist contact and learns that Buschmann no longer exists, having allegedly become a Trotskyite traitor. The contact forces Wolters to take payment from him, something he had always refused to do. Wolters tears up the money.

1949. Wolters is still working on Kettering's project, which, now in peacetime, is researching the medicinal uses of radiation. Kettering is visited by Thomas Northon of MI5, who is investigating the apparent presence of a traitor on his staff. Kettering is shocked to hear that the traitor is Wolters and agrees to aid Northon in his attempt to collect evidence for a prosecution. When interviewed, Wolters denies his treachery.

Later, Wolters is driving with Northon when a shot smashes the car's windscreen. Stopping at a pub, they learn that it was nothing more sinister than a child playing with his father's gun. Wolters professes his love of Britain and how he wishes he could make it his home. Northon remains convinced of his guilt.

That night, Kettering argues with his wife and she admits her love for Wolters. Not understanding its meaning, Hjoerdis shows Kettering the note of Wolter's appointment with his Russian contact from America. Recognising it as crucial evidence, Kettering drives off to deliver it to Northon, but crashes his car and is killed. The note is destroyed.

Learning from Northon that Hjoerdis attempted to stop Kettering from delivering the evidence before he departed, Wolters decides to confess his guilt. At Wolters' request, Northon meets Hjoerdis to explain the situation, and he says that Wolters may be released in ten years.