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Donnie Fatso is the ninth episode of Season 22. Jon Hamm guest stars as an FBI agent and Joe Mantegna returns as Fat Tony for the last time. This is the first time Fit Tony, Fat Tony's sporty cousin, appears.
Homer is sentenced to ten years in jail and in order to reduce his sentence the FBI offers him a deal, so he agrees to infiltrate Fat Tony's mob, undercover.
Homer and Marge wake on New Year's Day with hangovers after the family's New Year's Eve celebration. As Homer takes the garbage out, Chief Wiggum, Eddie, and Lou arrive and issue him multiple citations and fines - the result of recently passed, frivolous laws intended to bring in revenue for the city when broken. Taking Moe's suggestion that he bribe a city official to clear up the fines, Homer leaves a sack full of cash on the official's desk but is promptly arrested, convicted, and sentenced to ten years in prison. Wiggum takes pity on Homer and tells him to meet with an FBI agent, who offers to reduce the sentence if Homer will go undercover in the prison to investigate Fat Tony, who is also serving time along with his top henchmen.
Homer quickly gains favor with Fat Tony, due to a confrontation engineered by the FBI agent, and Fat Tony breaks him and the entire group out of prison and offers him a chance to join the syndicate. Homer's first task is to burn down Moe's Tavern in revenge for Moe's rudeness toward Fat Tony on the phone, but Homer finds that Moe has already done the deed himself. Fat Tony accepts Homer into the syndicate and the two develop a special bond; however, complications over a scheme to import weapons put the syndicate under severe stress. Eventually Fat Tony discovers Homer's undercover status and, emotionally devastated by his betrayal, suffers a fatal heart attack.
Meanwhile, Marge has begun to panic over being unable to communicate with Homer, as she knows nothing of his undercover work and cannot get any information on his whereabouts. She is surprised and thrilled when he returns home with his prison sentence lifted, but Homer feels guilt for Fat Tony's death and bitterness toward the government over being used to bring him down. Homer visits Fat Tony's grave to apologize, but is kidnapped by his cousin Fit Tony, who plans to kill him for revenge. However, Fit Tony spares his life after Homer tells of the time he and Fat Tony spent together, seeing that Fat Tony lives on in Homer's memories. Fit Tony takes charge of the syndicate, but the stress of the position causes him to overeat and gain weight, becoming known first as Fit Fat Tony and then simply Fat Tony - now physically indistinguishable from the original.
In court, Homer is sentenced to ten years in the federal penitentiary. He is given one option to get out sooner: He must wear a camera and microphone in a disguise and infiltrate Fat Tony's gang. In a parody of the award-winning movie Donnie Brasco, with elements from Tony, eats, gaining weight. He is then called Fit-Fat Tony, but gains more weight and is just called Fat Tony. He looks, acts and sounds like Fat Tony, which is The Simpsons way of making it like it never happened.