Fabienne, a young French-born British schoolteacher, marries and heads to Bucharest in the Eastern Bloc for their honeymoon. Her husband, Isaac Abela, is arrested by secret police and soon turns out to have been detained by Soviet intelligence as a spy. She plans to travel to Moscow to plead for his release, but a seemingly suspicious waiter drugs her and she wakes up on a plane bound for England.
Back in London, Fabienne seeks help from her uncle, the Shadow Foreign Secretary, and his ally, the head of British Intelligence. They reveal a hard truth: her husband was arrested as a bargaining chip to exchange for a top Soviet agent who has recently been unmasked by the British. What Fabienne does not know is that her husband, a corrupt businessman, is secretly in league with Soviet intelligence and married her on their orders. The planned exchange in Germany goes disastrously wrong when the Soviet spy, weighed down with Western consumer goods, sinks through the ice and drowns.
With no available hostage to swap, Fabienne takes matters into her own hands and begins to track an enemy agent. She corners a man in a trench coat who has shadowed her all day, only to discover he is not a true spy but an incompetent British agent ordered by his chiefs to keep an eye on her for her own security. In the midst of this, Andrej, the Bucharest waiter who reappeared rummaging through her room and pressing for a microfilm he planted in her luggage, returns to the foreground. Fabienne decides he could be the ideal exchange piece, but before she can act they are abducted again by enemy forces and manage to escape through the Scottish countryside.
Andrej reveals a startling twist: he is not a professional spy, but a man who smuggles manuscripts of dissident Soviet writers to the West. Because the Soviets want to recapture him, they plant evidence in his hotel room to portray him as their asset. Special Branch arrests him and agrees to a new prisoner exchange, even as Fabienne protests the move.
The climactic sequence unfolds on a lake near the East German border, where the proposed handover spirals into chaos and a high-stakes motorboat chase. Loyalties blur, seasoned operatives improvise, and Fabienne confronts a brutal reality: the dream of reuniting with her husband is tangled with political games, espionage, and the personal cost of defiance.