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American gambler Dan Shane, who has been left homeless by the German aerial bombing raids, breaks into a seemingly empty London home for protection from the night rains.  Leslie Stafford discovers the intruder in her house and, rather than calling the police, she offers to stake Dan his fare back to America if he will help her dispose of the body of her murdered husband, Captain Edgar Stafford.

Pretending to be drunk, Dan stuffs Edgar's body into a taxi and then leaves it in a waterfront phone booth.  Dan then returns to Leslie's, and even though he believes that she is a murderer, propositions her.  She orders him to leave, but runs after him when her husband's body reappears in the den.

When the police arrive, the two manage to escape in a car parked by Leslie's back door.  Dan and Leslie then travel to Scotland, where they are forced to stay overnight at the Fisherman's Rest Inn under the aliases of "Mr. & Mrs. A. Cooper."  That night, Leslie tells Dan that her estranged, abusive husband returned to her the night of his murder and made "wild accusations" against her cousin, Lord Abbington, to whom she was once engaged.  When Edgar refused to leave, she went to bed, only to be later awakened by the cries of her husband, whose dying words were "S I 10."  Dan does not believe her story, so Leslie pretends to send his ticket home to her cousin's castle in order to force him to accompany her there.

Along the way, they bypass a hitchhiking German spy, Carl, who notes that the license plate on their stolen car is "S I 10."  After leaving Leslie at Lord Abbington's, Dan motors back toward London, but stops when the car receives a German shortwave message for "S I 10."  Dan then discovers a hidden shortwave radio in the spare tire compartment and sees the license number.

He goes back to the Abbington estate, where he pretends to be a newspaper reporter in order to question Lord Abbington.  Abbington tries to bribe Dan, then discovers the American's true identity from a newspaper story.  Abbington is about to turn Leslie and Dan over to police inspector Robbins when the policeman points out that the lord's missing car has been returned.

Later, Dan escapes with Leslie from the lord's home, and the two are forced to seek refuge in a barn after Leslie sprains her ankle.  Leslie tells Dan that her cousin was an adopted German orphan whom her husband suspected of being a Nazi operative.  Back at the Abbington estate, the lord chastises German agent Hans for his failure to frame his cousin for her husband's murder.

The next day, a search party, led by Inspector Robbins, uncovers Dan and Leslie, but the policeman mistakenly leaves Leslie in the hand of Abbington's men.  She is taken to Abbington's waterfront warehouse, where she overhears the lord telling Carl that he is placing explosives in whiskey bottles which will explode during shipment at sea.

Meanwhile, Dan tells the suspicious police that Abbington is a Nazi spy who ordered his agents to kill Stafford.  The lord then arrives in town and is able to refute Dan's entire story.  Dan escapes on a motorcycle, with the police and Abbington giving chase.  He arrives at the warehouse, where he saves Leslie from Hans's clutches.  Before they can escape, Abbington appears, so Dan threatens him with one of the whiskey bottles.  The spies surrender, but rather than being congratulated for their efforts, Dan and Leslie are chastised by Scotland Yard for their interference in the case.