Underworld tells the tale of William Smith, a middle-class school teacher from a suburb in London, who returns home one day to find his wife Gilda has run off with her boyfriend and taken all his possessions with her. With the help of his sister, Susan, they begin a search for Gilda, and are drawn into the British crime world and become caught in the cross-fire of two crime lords who have been fighting each other all their lives: the kind-natured Teddy Middlemass, and the sadistic Mr. Jezzard.