Susanne Mallison flies to Berlin to visit her newly married brother, Martin, who works as a doctor for the British Army. Martin's wife, Bettina, meets her at the airport. As they leave, a young boy follows them on a bicycle. Bettina takes Susanne on a short drive through Berlin, explaining that it is still struggling to rebuild itself after the war. By the time they arrive home, Susanne notices that Bettina is frequently agitated. When her brother comes home, the three go out for dinner.

Later that evening, at a club, Susanne notices Bettina deliberately spilling a drink and pretending to go to the washroom. In a mirror she watches Bettina become increasingly distressed as she speaks to someone hidden from view. When Bettina says that she has a headache, the three return home. That night, Susanne sees Bettina take delivery of a letter from Horst, the boy on the bicycle. Bettina later denies that anyone came to the house.

The next day, Bettina takes Susanne on a tour of East Berlin and there she meets Ivo, an old friend. After visiting her brother, Susanne comes home and overhears Ivo and Bettina in an animated conversation in German. Although Susanne can't understand their words, it is clear that Bettina is extremely upset. When they realise they are not alone, they pretend that nothing has happened. Ivo offers to take Susanne sightseeing. While at a club, they are approached by Halender. Ivo is clearly unhappy to be disturbed, but sends Susanne home so that he can speak with him.

The next days Susanne meets Ivo, who apologises for sending her home. He tells her that he feels guilty about his relationship with Bettina and that he plans to leave Berlin and leave her alone. Ivo and Susanne plan to go ice-skating. She stops at home to pick up a pair of skates. There they meet Kastner, who Martin is treating for a dog bite. At the ice rink, Halender blackmails Ivo into helping him stop Kastner, who has been smuggling people out of the East. Ivo asks Susanne to set up a meeting with Kastner, pretending that he might be able to get him letters of introduction to leave Berlin.

Bettina is furious when she hears of this and reveals that Ivo is a criminal involved in kidnappings. Susanne thinks that Bettina has been having an affair with him, but Bettina confesses that she is actually Ivo's wife. They had married before the war, but he had been reported dead in France; he reappeared after she married Martin. Bettina is sure that Ivo plans to kidnap Kastner. They go to the authorities and convince Susanne to set up a meeting so that they can arrest the kidnappers. However, Ivo realises it is a trap and doesn't appear.

Halender's men, mistaking Susanne for Bettina, kidnap her and take her into the Eastern sector. Ivo convinces her that if she refuses to act as bait to catch Kastner, she will be killed. They arrange to meet at the Opera House. Ivo, however, has made arrangements with two confederates and escapes with Susanne. They find all the borders heavily patrolled, and so go to find Horst's grandmother, who provides food at a building site. They find shelter for the night, after which Horst is sent to deliver a message to Martin and Kastner. Ivo tells Susanna that he is a bad lot. He was a lawyer before the war and was then forced to take part in many atrocities as a soldier. Susanna says that she loves him and the two pass the night together.

Kastner arrives the next morning and transports them to the border in the back of a laundry van. The guards initially let the van pass, but become suspicious when they see Horst hanging around on his bike. Ivo makes a break for it to distract the guards and is shot just as Susanna and Kastner manage to cross the border into safety.