A serial killer is on the loose in 1946 London with five victims so far. Elderly Mrs. Thompson rents a room to a secretive young Tim Matthews and is convinced he's the murderer. She calls police after discovering a knife in his room. Meanwhile, Tim has taken a young woman he's obsessed with on a picnic in the country. Having been discharged from the Royal Air Force for physical reasons, he's insanely jealous of her former boyfriend, a strapping man who had served during the war.