When impressionable Mary Tillet loses both of her parents and her house during a bombing raid on London, she is given shelter in the home of Jack Rawlings, who lives in the apartment over his tobacco shop. Despite Rawlings' outward kindness, there is something about him that makes the young woman nervous. Her fears are well-founded, for Rawlings is actually Dr. Vernon, who has been wanted for the murder of his wife since 1925. Rawlings is also responsible for a current spate of murders, all of which he committed during blackouts using a poisonous drug administered with a hypodermic needle hidden in his pipe. After Rawlings kills Hendrick Peterson, a foreign industrialist, Mary reads a newspaper account of the crime and begins to suspect Rawlings, for she has accidentally seen the needle in his pipe. Mary tries to show it to her fiancée, Dutch soldier Peter Dongen, but Rawlings has removed it and Peter thinks that she imagined it. Meanwhile, Superintendent Neil and Inspector Harris of Scotland Yard question Peterson's partners, Oliver Madison and Eugene Caldwell, and are surprised by their unwillingness to help with the case. Back at Rawlings' flat, Mary's nervousness increases when he confesses that he used to be a doctor, and therefore would have the medical knowledge necessary to be the perpetrator. Wanting to calm her down, Rawlings persuades Mary to take a sleeping powder, then leaves. He takes a train on which Caldwell is a passenger, unaware that Harris, who has been trailing Caldwell, is also on board. During a blackout on the train, Rawlings injects Caldwell with the poison, then leaves before he succumbs. Harris goes to see Rawlings in the tobacco shop the following morning and accuses him of being the killer. Rawlings thwarts him, however, by showing him that morning's newspaper, which has a headline that the blackout murderer, George Sandley, had been shot to death by Madison during an attempt on his life. Actually, George was Rawlings' devoted assistant and confessed to the murders before he died to draw suspicion away from Rawlings. Harris is not convinced and steals Rawlings' newspaper, which has his fingerprints on it, and thereby identifies him as Dr. Vernon. During Rawlings' trial, he readily confesses to killing his wife and says that he did so because she was a wicked woman who drove two of his friends to suicide. Rawlings then asserts that he killed his most recent victims because they were participants in a secret German plot to destroy English unity with a phony peace proposal. The judge questions Madison, and upon confirmation of Rawlings' claim, orders him to be arrested. Although the judge then expresses sympathy for Rawlings' motivation, he states that murder cannot be condoned under any circumstances and sentences Rawlings to be hanged. As Rawlings is being led from the courtroom, he hands his pipe to Peter and Mary, in case they want to make use of it. Peter destroys it, however, and the relieved couple embrace.