Story follows the rise and subsequent fall of the notorious head of a New York crime family, who decides to testify against his pals in order to avoid being killed by his fellow cohorts.

In New York City, a gangster kills a fellow hoodlum for personal vengeance and consequently violates the unwritten law of the syndicate. The punishment for personal revenge that draws unwanted attention to the organization is death. Charles Lupo, the area boss, gets permission from his superior in Italy and then hires Nick Magellan from Chicago to accomplish the execution of the errant mobster, believing that Nick's anonymity in New York will make the killing go smoothly. With calm precision, Nick kills the transgressor and so impresses Charlie that he arranges to have the younger man transferred to his territory. After a murder attempt is made on Charlie, Nick serves as his bodyguard. While checking the grounds around the Lupo mansion, where Charlie lives with his mother and daughter, Nick spots a couple in a parked car and orders them out at gunpoint. The woman, who reveals that she is Charlie's daughter Katherine, slaps Nick and introduces the man as her boyfriend Stan. Inside the house, Kathy expresses her frustration at being the daughter of a mobster. She resentfully laments that, although Charlie sent her to finishing school, his occupation scares away "decent people," including Stan, who has decided that her mob connections will jeopardize his future. When Stan later tries to reconcile with her, Kathy, correctly guessing that he has accepted a bribe from Charlie, rejects him and decides to leave home. The gangland slayings attract the attention of politicians, who appoint Judge Kincaid to head a cleanup campaign to expose the syndicate. Charlie learns about the crime commission and makes plans to bribe its key members. Immensely pleased with his new employee's intelligence and loyalty, Charlie grooms Nick for success within the organization. When Charlie receives an emergency call cutting short a date with his mistress Iris Palmer, he asks Nick to take her to dinner and the theater. From that evening on, Iris tries to seduce Nick, but is politely refused. When Charlie's colleagues worry that the commission's investigation will jeopardize an oil deal, in which they have already invested $10,000,000 for legitimate expenses and bribes to Washington lobbyists, Charlie reminds them that everybody has his price. Charlie, with whom Kathy has ceased all communication, worries about her, but his mother begs him to leave her alone. Having witnessed the cycle of in-house killings over the years, Mama Lupo senses that bad times are ahead within the organization. At Charlie's request, Nick traces Kathy to a doctor's office where she uses a false name in her work as a receptionist. Over dinner, Nick, who grew up in poverty, suggests that the organization is no more corrupt than the rest of the world and advises her to accept what she has. When she insists that she is content with her new life, he agrees not to tell Charlie where she is, but warns her that it is unlikely that she can escape her situation. Although they seem compatible, both resist pursuing their relationship; Kathy because she wants to be free of the organization and Nick out of loyalty to his boss. When Charlie's best friend and right-hand man, Ben Dagajanian, is arrested for illegal entry into the country, the bosses decide that they are unable to assist him without endangering the syndicate and choose not to interfere with his deportation. Later, Paul Williamson, a lobbyist who accepted their bribe money, double-crosses them, and the key syndicate chiefs from all over the country vote unanimously to assassinate him. Charlie has disturbing premonitions about this "hit" and decides that Nick is now too important to risk, so three men are assigned to kill Williamson. During the bumbling execution of their assignment, the men kill a policeman, who dies after identifying them. To prevent their apprehension by the police, Nick is sent to kill the three men, who have scattered and await instructions. He efficiently kills two, but the third, Arnie Wendler, escapes and turns himself over to Kincaid, offering to turn state's evidence and expose Charlie. Later, a drunken Kathy comes to Nick's apartment and tells him that she was fired after newspaper reporters uncovered her identity. She then throws herself on Nick, and asks him to marry her. Although Nick sympathizes with Kathy, he rejects her proposal and warns her that she is close to a breakdown. Kathy, depressed, leaves the apartment and later dies in a car accident, which the police determine was a suicide. When their inside sources report that a "Kefauver-like" investigation is being planned, the syndicate leaders decide that Charlie must be sacrificed to relieve pressure on the organization. Understanding that the organization comes first, Charlie prepares to be tried in Washington. When it is rumored that Charlie is planning to betray them, the syndicate leaders vote to execute him before he leaves for Washington. Knowing that Nick, who thinks of Charlie as a father, can get close to Charlie, the bosses choose him as executioner, despite his request that they choose another. After Nick leaves, the bosses make additional plans to eliminate him "as insurance." Nick meets with Charlie, who guesses his purpose and resigns himself to dying. With regret, Nick says goodbye to Charlie and then shoots him and Iris. Shortly afterward, he, too, is killed by his colleagues for knowing too much.