Young Bill Gridley, an American State Department official, is transferred to London, and he rents a room in a stately old town house belonging to American Carlye Hardwicke. The next day he learns from his superior, Franklyn Ambruster, that Scotland Yard suspects Carlye of having murdered her husband, a Britisher who is reported to be missing. Bill refuses to believe the accusation, though he reluctantly agrees to spy on his beautiful landlady. Then one night, he hears sounds of a scuffle and races into Carlye's room to find her holding a revolver over the lifeless body of her husband. At the coroner's inquest, Carlye is vindicated by the testimony of Agatha Brown, a private nurse who cares for Carlye's crippled next-door neighbor, Mrs. Dunhill. The nurse claims that she saw Carlye struggling with her husband and that the murder was committed in self-defense. What she does not tell the court is that the husband had become enraged when he discovered that Carlye had innocently pawned a candelabra in which he had hidden a number of stolen jewels. Following the trial, Mrs. Brown murders the pawnbroker, obtains the jewels, and goes to a seaside resort to kill old Mrs. Dunhill, who also knows about the jewels. Carlye and Bill learn of Mrs. Brown's intent and take frantic pursuit. Mrs. Brown pushes Mrs. Dunhill's wheelchair down a winding cliff edge, but Bill makes a wild dash and catches it just in time. A flying tackle by Carlye subdues Mrs. Brown as Ambruster and Scotland Yard's Inspector Oliphant arrive on the scene by helicopter.