Hailed as a hero for his efforts against a group of racketeers, F.B.I. agent Jim Conway is asked by District Attorney Edward Greer to become an assistant district attorney in his office. Unaware that Greer desires him for political reasons only, Jim, who graduated from a night law school, eagerly accepts the position. When Jim learns about Greer's true intentions, however, he threatens to quit but changes his mind after he is assigned to represent Letty Lane, a wild debutante whose older sister Pat, Jim admires. While Jim romances Pat, the racketeers, led by Whitey Clark and Chin Martin, press the local businessmen to put slot machines in their shops. Concerned by the shop owners' resistance to his scheme, Chin Marin hires lawyer Steve Lawrence, Letty's boyfriend, to file an injunction against the district attorney's office. Lawrence also suggests to Chin that he would provide better leadership for the gang than Whitey, who uses excessive strong-arm tactics, and Chin agrees to consider Whitey's elimination. Whitey then orders his men to "put the heat" on a select number of uncooperative shop owners, and Franz and his young son Otto, neighbors of Jim, are attacked. After Otto dies from his wounds, Jim demands to take the case against the hoodlums, but is unable to convict any of the indicted men because of judicial corruption. Determined to nail Otto's killer, however, Jim and police detective Mac trick a fourth thug into identifying the attackers. After the ambitious Chin and Lawrence arrange for Whitey to be killed during an attempt on Jim's life, Jim, who has been named a special prosecutor, learns that the gang has infiltrated the entertainment business. By convincing Flo Fisher, nightclub owner, to "play dead" in front of her fellow entertainers, Jim secures enough testimony to indict a key racketeer. Lawrence, meanwhile, sets up the greedy Chin in a police raid and leaves for the country without Letty. Suspicious, Letty follows Lawrence to his country home and, when Chin arrives and threatens Lawrence with a gun, kills the interloper while Peggy, Lawrence's other woman, watches. Eventually Jim deduces that Lawrence is the "big boss," but is stymied when he accuses Letty of Chin's murder. Armed with Peggy's eyewitness account, Lawrence tries to bargain with Jim for a reduced charge but is denied. To save Pat from scandal, Letty then kills herself in an automobile crash. Later, Jim, who is now married to Pat, opens his own law office.