After having a particularly bad day at school, Bart gets caught in a downpour and loses control of his skateboard. He crashes down the stairwell of the Legitimate Businessman's Social Club, a Mafia bar owned by the Springfield Mafia. Fat Tony, the leader of the Mafia, and his henchmen Legs and Louie are initially inhospitable towards Bart, but they are impressed by his ability to pick the winning horse in a horse race on television. When Bart makes up an excellent Manhattan cocktail for the mobsters, Fat Tony hires him as their bartender and errand boy. As Bart adopts more and more gangster-like traits, Marge grows anxious and tells Homer to go and meet the Mafia, but Homer approves of them after they let him win at poker.
When Fat Tony finds out the Principal Skinner is giving Bart detention after school, the mobsters go to confront Skinner. The next day, Skinner is missing and is presumed to have been murdered. Bart has a nightmare of Skinner's ghost and his own execution on the chair. He rushes to confront Fat Tony at the bar. When the police burst in, Fat Tony blames Skinner's disappearance on Bart, who is put on trial. During the trial, Fat Tony, Legs, and Louie lie to the court, saying that Bart is Skinner's murderer and the leader of the Springfield Mafia.
Judge Snyder is about to convict Bart when Skinner bursts through the doors, disheveled and unshaven, and explains that neither Bart nor the mobsters assaulted him. Rather, when Fat Tony and his henchmen had come to his office meekly, Skinner had yelled at them to not interfere with student discipline, and they left sheepishly. After Skinner returned to his house, he became trapped beneath tons of old newspapers in his garage, and was stuck there for over a week, until he was able to escape. Bart is cleared of all charges despite counsel for the prosecution unsuccessfully trying to convince the Judge to strike Skinner's speech from the record. Fat Tony tells Bart that he did not want to get him into any trouble on his behalf, but Bart decides to resign after learning that "crime doesn't pay".