A man helps the authorities uncover a ring of murderous German spies in wartime London.

During the dark days of World War II, young Mary Tillet is bombed out of her home and forced to seek lodgings upstairs from a tobacconist. Her kindly but mysterious landlord, Jack Rawlings, attempts to transform the empty, web-strewn apartment into a home. After reading newspaper accounts of the "London Blackout Murders," Mary is plagued by dread. Is her landlord a hero, or a sinister assassin? Taken as a period piece, the movie is a gem that briskly portrays the effects of wartime shortages, women in the workplace, and the moral ambiguity inherent in life-or-death situations.—plot summary by floodmouse