Undercover agent Nick Stone infiltrates a gang of jewel thieves to find evidence that a man convicted of murder during a jewel robbery is actually innocent.

The agent joins the gang of William Kane, their objective is to run the leader of the numbers racket out of town.

The undercover agent joins in on a robbery.

After a boxer is killed for not throwing a fight as ordered, Nick goes undercover by taking over as manager for another young prizefighter, planning eventually to get him to turn evidence against the crooked promoter who ordered the killing.

Barney MacCready puts his train-robbery plan into action. First he kidnaps a detective, then he has two of his henchmen board the train one posing as the missing detective the other as his prisoner.

An undercover agent assumes the identity of a killer hired to rub out a murder witness. But the intended victim is hiding, and the first task is to smoke him out.

Sam Keely and Jeff Stewart are a bit on the shady side. The undercover agent's strategy is to muscle in on their racket.

Dan Hawley and David Johnson, witnesses to a murder committed by gangster Jimmy Farrow, are a target for Larry Maddox, a hired killer. After Maddox kills Hawley, police take Johnson's wife and children into protective custody. Johnson, fearing for his life, goes into hiding. Nick is put on the case.

Mobster Sid Zero is a man who gets what he wants in business and women by using muscle, threats, and, if necessary, murder. Nick goes undercover to bring him down, and tries to get the help of the latest young model Zero has forced into becoming his mistress.

A detective assigned to investigate a phoney health club is killed by one of the club owners. An undercover agent follows up, convincing the gangsters that he's a gambler on the run.

Underworld leader Max Rommey wants Paul Anderson dead. To protect Anderson, the undercover agent becomes part of Rommey's gang.

The first step for Nick in infiltrating Vince Baron's gang is foiling the attempted kidnapping of Baron's daughter. He must also contend with a young upstart who wants to push Baron out as leader. What Nick was not expecting was to fall for the daughter.

The undercover agent is assigned to break up a gambling ring that bets heavily on certain horses--and then makes sure they win by doping them.

The undercover agent is assigned to smash a dope ring that sells to Americans from a base in Mexico.

Nick is assigned to break up a gang headed by Steve Taylor. This particularly unsavory gang uses threats and violence to force small shopkeepers to pay for protection.