The Cotton Club in London's Soho district is operated by American gangster Steve Marco who, when Joe Lane threatens to tell the police of his past, has no qualms about killing him as he figures if he could outwit America's G-Men, he has little to fear from Scotland Yard. Joe's body is found in Greek Street, and Inspector Hammond questions everybody employed at Steve's club where Joe was last seen. The investigation yields nothing until dance hostess Ruby Lane, wife of the murdered man gives away a vital clue - a priceless string of pearls he had been teasing her with that Hammond decides is now in possession of the killer. Ruby, acting on orders and with Steve watching her, breaks a string of imitation pearls which Steve promptly offers to replace, much to the chagrin of newspaper reporter Roy Barnes who is in love with her. Steve takes Ruby to dinner at his flat where he gives her the promised pearls. Later, at the club, she gives them to Barnes to give to Inspector Hammond but is seen by two of Steve's henchmen. They follow Roy, take the pearls away from him and leave him unconscious in a taxi. Steve refuses to believe that Ruby is double-crossing him, but agrees to put her to a test.