Married couple Fabienne (Marlene Jobert) and John (Patrick Mower) are honeymooning in Bucharest when the latter is arrested by the secret police. The anxious Fabienne is forced to return to Britain without him. Meanwhile, it is revealed to the viewers that John is actually a spy working for the Russians, and that his arrest was just an elaborate deception created so that they could contact him. Fabienne is duped into believing that John is now a hostage, his only hope of release being that the British secret service might release one of their Russian prisoners in exchange for him. Still unaware of the double-cross, Fabienne works tirelessly with her uncle Sir Trevor (Trevor Howard) - a foreign office diplomat - to get the British government to trade a Russian spy for her husband. Just when it seems that a deal has been struck, the spy they choose to bargain with is accidentally killed. Fabienne instead sets her sights on Romanian spy Andrej (Kirk Douglas), but as she tries desperately to trap him in order to use him in the exchange, she discovers herself to be falling in love with him. All is resolved in a last-minute revelation at an exchange-rendezvous-point near the Iron Curtain.