Billy Simpson is a shady boxing promoter in South London banned from legitimate fights. It's the biggest day of his life, though, because he has a champion on his hands, his 20-year-old son, Eddie. Billy's put all his money on a victory, as have his quarreling daughters. But Eddie is nervous, an odd guy in a cap is hanging about, the licensed promoter Billy has hired to arrange the evening's other fights has hired dogs, the manager of Eddie's opponent looks down scornfully on Billy and on South London, and Scotland Yard wants to Billy in gaol for the death of a club brawler. When the night goes disastrously wrong, Billy tries to find out who double-crossed him.