The local mob is killing off the small operators who are horning in on their gambling business. Nick flashes some cash and gets invited to one of the mob's casinos where he plays like he's a tough guy. This catches the boss' eye because he's always looking for new thugs.
Nick works undercover on a bank robbery. After the thieves are apprehended, the $100,000 they stole is nowhere to be found, and the police lieutenant who apprehended them is accused of taking the money. Now Nick works to prove the lieutenant innocent.
Nick wants to get hired by a gang planning on cleaning out the vault at a jewelry store. He makes sure he's seen as a flashy thug who needs work after a dame (who works for them) slips him a mickey and cleans him out.
A mobster just released from prison finds that his former protege now controls half of his old territory. Nick goes undercover to infiltrate the ranks of both men in order to get the evidence against them and prevent a major gang war from erupting.
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.
A police detective vanishes after he is accused of robbing a train and killing two employees. Nick is sent undercover to determine if the officer is really guilty, or if he is even alive, and bring whoever is guilty to justice.
After a polished blackmailer drives one woman to suicide, Nick goes undercover to stop the man from his next target, actress Laura Crane. The blackmail scheme involves the suspicious death of Laura's first husband.
A shoe shop owner witnesses a mob murder, and Nick assumes the identity of the hit man who's been sent for to kill him. But the witness goes into hiding, and so the mob decides to smoke out the old man by using his wife and brother. Nick must act carefully to protect all three.
Posing as a businessman representing an electronic firm, Nick gets in with a mayoral candidate who's part of the mob. He offers Nick some free city land if he'll help frame the current mayor with a phony bribery set-up.
One of two witnesses to a mob killing is murdered. Though Nick has had all he can take of undercover work and requests being taken off of it, he is assigned to infiltrate the mob to protect the other witness, who has now gone into hiding.
While undercover on a payroll robbery in which a cashier is killed, Nick is shot by a policeman. Though suspicious of him, the robbery gang takes him to their secret hideout, where he must continue walking the tightrope to avoid giving away his identity and yet stop other innocent people from being killed.
A diamond cutter is robbed of a prize stone, but refuses to cooperate with police in identifying the thieves, because he recognized one as someone close to him. He is thus suspected of involvement with the thieves. Nick goes undercover to learn the truth.
Nick is sent undercover to protect Paul Keel, a witness against a mobster who has now been released and is trying to kill him. But the assignment is complicated by the fact that Nick was once in love with Keel's wife, and she still has feelings for him, not knowing that he is an undercover man.
Frank Teal, a criminal operator who hides behind legit businesses, has someone on his hit list. The cops' informant is gunned down before they can find out the identity of the marked man . Nick gets himself hired by Teal, hoping to learn the name of the man to be murdered.
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.
After an undercover officer is murdered while investigating illegal betting, Nick goes undercover to find the murderer. He soon settles on two bookies, and puts himself on the line by placing opposing bets with each of them.
Family man Fred Anderson identifies a hired killer, who is then killed by police. To send a message to all potential witnesses, gang boss Max Romney orders Anderson killed. To protect him, Nick goes undercover as a man who wants to kill Anderson himself.
A wealthy man is murdered on the street at night in an apparent holdup. But it's actually part of a scheme to cheat the man's niece, who has been gambling in a house run by a crooked casino owner. Nick is sent in undercover to put a stop to the operation.
Nick goes undercover as a jewel thief to bring down mob boss Ben Verlaine, not knowing of an even greater danger in the same town---an old associate from two years earlier who has figured out that Nick was the one who sent him to prison, and now wants revenge.
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.
A witness to a murder committed by a man posing as a cop is so scared, and distrusting of the police, that he decides to hire his own gunman to act as his bodyguard. Nick sets things up to get the job and find the killer as well as protect the witness.