In the Dorset countryside, Robert Taine shoots at a man he believes is a poacher on his rented property. The man named Reimann [Denis Lehrer] is mortally wounded, though [Hiart] [Marius Goring] simultaneously shot Reimann. Unaware of Reimann's death, Taine hides Reimann's body under a shrub as he later encounters Hiart and his driver, Diss [Karl Stepanek], who are looking for the body.
That night, Hiart's associate Magda Hassingham [Patricia Laffan] discusses the incident with him. Magda's drunkard husband [Frank Lawton], the major landowner in the area, is ignorant of her involvement with enemy agents. Intending to bury Reimann the next day, Taine finds Peter Sandorski [Herbert Lom], a Polish operative for British military intelligence. Sandorski enlists his help with a secret operation to foil a gang out to steal atomic secrets. British Colonel Cartwright [Cyril Raymond] introduces Taine and his wife Cecily [Evelyn Keyes] to government official Randall [Roland Culver], who confirms Sandorski's identity and story and instructs Taine to cooperate with Sandorski.
When Randall leaves, Taine confesses to Cecily that he has accidentally killed a man. Cecily insists on accompanying her husband and Sandorski to a nearby field where an enemy agent is expected to land an aircraft. While Cecily stays in the car, Taine and Sandorski see Hiart, Magda and their assistants arrange landing beacons. When he overhears that the agent, identified as Lex [David Hurst], will only be in town for 48 hours, Sandorski decides to capture him. By moving a beacon, the landing is disrupted, allowing Sandorski who is impersonating Hiart to grab the spy. Sandorski tosses grenades so they cannot be followed.
While Taine and Cecily impersonate the Hassinghams, Sandorski drugs Lex so he will sleep. The next morning police discover Reimann's body. Diss, meanwhile, attempts to locate Lex and learns about Taine and Cecily's unusual houseguests from their housekeeper's son, Tommy Powell [Robert Dickens]. After police inspectors Matthews [Jack McNaughton] of Dorchester and Sullivan [Clement McCallin] of Scotland Yard question Taine and take one of his boots to compare to the prints found near Reimann's body, Taine insists they all leave for London, where Lex has a meeting.
On Taine's instruction, Mrs. Powell [Megs Jenkins] contacts Randall with a request to call off Scotland Yard. Hiart pursues Taine, Cecily, Sandorski and Lex who arrive at a train station where Randall makes contact with Cecily, who admits that her husband killed Reimann. While on the train to London, Cecily takes Lex's briefcase while he is asleep. At the London train station Hiart manages to alert Lex, who escapes. However, the next day at London's Madame Tussaud's Wax Museum, Randall and Sandorski arrest Lex after he passes an envelope to an unidentified spy. Lex and the spy are arrested, but Hiart and Diss flee.
Sandorski shoots Diss during a pursuit in a stairwell, and Hiart is killed when he opens Lex's briefcase, triggering an explosion. Randall later reveals to Taine and Cecily that Lex, a scientist, was brought to England to interpret stolen reports about British atomic weapons trials in Australia. He and Sullivan then inform a relieved Taine that it was Hiart who killed Reimann. Before departing, Sandorski cautions Cecily that any future strange behaviour by her husband may be related to espionage.
