Plot
A U.S. Army colonel Robert Taine living in the English countryside shoots at a man he takes to be a poacher on Taine's rented property in Dorset.
The man named Reimann, has been mortally wounded but Robert is unaware that a foreign spy named Hiart simultaneously shot Reimann. Believing he has killed the poacher, Robert hides Reimann's body under a shrub. Robert encounters Hiart and his driver, Diss looking for the body.
Later that night, Hiart's colleague, Magda Hassingham, discusses the incident with him. Magda's drunkard husband, the major landowner in the area, is ignorant of her involvement with enemy agents.
Intending to bury Reimann the next day, Robert finds Peter Sandorski, 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 introduces Robert and his wife Cecily to government official Randall who confirms Sandorski's identity and story and instructs Robert to cooperate with Sandorski.
When Randall leaves, Robert confesses to Cecily 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, Robert and Sandorski see Hiart, Magda and their assistants arrange landing beacons.
When he overhears that the agent, identified as Lex, 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 Robert 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 Robert and Cecily's unusual houseguests from their housekeeper's son, Tommy Powell (Robert Dickens). After police inspectors Matthews of Dorchester and Sullivan of Scotland Yard question Robert and take one of his boots to compare to the prints found near Reimann's body. Robert insists they all leave for London, where Lex has a meeting.
On Robert's instruction, Mrs. Powell contacts Randall with a request to call off Scotland Yard. Hiart pursues Robert, 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 Robert 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 Robert 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.
