Tony Anderson, an academic attending a conference in Prague during the Cold War era, is mistaken by security chief Alexander Placek for an expected western emissary to illicit dissident groups. In order to undermine such activities he sends Anna Jirak to worm her way into Tony's household as a spy. This she does by posing as a political refugee needing a form of marriage to the now-widowed Tony in order to escape deportation.
In time the marriage becomes genuine, so when Placek turns up demanding that Anna should wreck Tony's standing by accusing him of multiple adultery with supposed refugee women, she refuses. Placek's unwelcome duty then requires him, in the course of offering Tony a session chairmanship in a forthcoming conference in Kiev, to denounce Anna as a traitor to her own family and friends. Unwilling though Tony is to believe the story, it has enough verisimilitude to create a doubt that she cannot dispel, and in despair she takes an overdose of medicine.
Determined to avenge her fate, but knowing that he must work with Placek at the Kiev conference, Tony consults his cousin Eric in the UK security service. Eric advises an indirect approach by joining an actual dissident support group. This proves to be badly run and Tony is persuaded to take charge of it.
In Kiev, Placek confides in his devoted assistant Elena about his distress at the news of Anna, whom he had regarded almost as a surrogate daughter. He instructs Elena to accompany Tony on the city tour when he expects contraband to be somehow handed over, but the plan is thwarted by local police who turn out to have been Tony's contacts.
Back home, some time later, Tony is visited by Eric together with Placek who, disgusted by new instructions from his masters, wishes to defect but needs the help of Tony's group for Elena to escape. Tony is reluctant but urged by Eric, for his own reasons, to comply. Anna, who is now revealed to have survived the suicide attempt at the cost of a partial breakdown, praises Placek's character and adds her own plea that convinces Tony to agree, and insists that the two couples should thenceforward be friends.