"Bart the Murderer" is the fourth episode of Season 3.
After having a very lousy day of school, Bart accidentally stumbles into 'the Legitimate Businessman's Social Club,' a mobster front. He becomes their bartender, but when he shows up late at work and blames Principal Skinner, Fat Tony then goes to confront Skinner. The next day when Bart comes to school, he discovers that Skinner is missing.
The day begins with Bart preparing for the school day, during which he is supposed to go on a field trip to the chocolate factory. But his day quickly turns sour, as Homer steals his cereal prize, Santa's Little Helper eats his homework, and he misses the school bus and arrives late for school. During recess, he rips his pants, and finally, he realizes he forgot his permission slip for the field trip, and is forced to spend the day at the school with Principal Skinner.
On his way home from school, he wipes out skateboarding in front of a local mafia hangout, and meets the Springfield Mafia Don, Fat Tony. After picking a winning horse for him, they tell him to mix drinks for them. Bart reads in a chart how to make a Manhattan, and makes Legs a "supoib" one, so they hire him as their bartender.
Not knowing that his new boss is a mobster, Bart begins to unwittingly help Fat Tony, accepts a gift from him, and even hides a stolen cigarette shipment in his bedroom. As he begins to flaunt his money, Marge becomes concerned and asks Homer to talk to Bart's employers and find out more about them.
Homer is completely oblivious, as he approves Bart's job after the mobsters intentionally lose at poker in order to curry Homer's favor.
Carrying over his new persona into the schoolyard, Bart gets detention for trying to bribe Principal Skinner. As a result, he is late for work (showing an in-universe depiction of the chalkboard gag), where Fat Tony has promised his mafioso guests the "finest Manhattans in all of Springfield." Bart arrives later, and is reprimanded by Fat Tony. When Bart explains that his absence was due to Principal Skinner, Fat Tony decides that he and his associates will go to 'meet and greet' this individual.
The very next day, Skinner goes missing. Bart suspects Tony may have something to do with it, and begins to have nightmares about Skinner's death. Bart runs to the mob headquarters, where they deny having killed Skinner. During their chat, Chief Wiggum and his officers enter Tony's hangout, arresting everyone in there for Skinner's murder.
A trial begins, but unfortunately Lionel Hutz acts as Bart's lawyer. On the stand, Bart is upset to find that Fat Tony and his gang name Bart as the Don of their organization, pinning any issues regarding Skinner's disappearance (as well as everything else they did) on him. Just when it seems that Bart is going to be found guilty, Skinner bursts into the courtroom, and tells everyone that he was in fact trapped under a pile of newspapers in his garage the entire time. The case is dismissed, and Bart angrily rejects Fat Tony for using him. The case of Skinner's disappearance becomes a highly inaccurate TV movie, but when Marge reveals it was so inaccurate the producers were not required to pay royalties to the Simpsons, Homer remarks that they should go after the real crooks in Hollywood.