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.
It's only 11 hours into the New Year and Homer has already gotten into trouble with the law. To recover government funds, a series of new laws have been passed that make almost everything a heavily fined crime, including telling it like it is and even adding up the total of his fines. On Moe's advice, Homer goes to city hall to bribe an official out of his debt, but his impatience leads him to leave a bag clearly marked 'bribe' in the window while he's away, and he's soon caught and arrested.
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 mic in a disguise and infiltrate Fat Tony's gang. In a parody of the award-winning movie Donnie Brasco, with elements from The Godfather and The Untouchables, Homer becomes very close to Fat Tony.
Fat Tony openly gives him his complete trust; meanwhile, Homer is transmitting to the FBI. Homer is torn between the law and his newfound friendship. Homer is there for Fat Tony, whether it is putting oregano on his spaghetti and meatballs or visiting his late wife's grave who was "whacked by natural causes". Near the end of the episode, following the line of Donnie Brasco, there is a big bust. Homer tells Tony to get out of the area, but because he is so confused, Fat Tony hesitates. The FBI comes in. In a tragic display of emotion Fat Tony tells Homer that he was his friend and he trusted him. He then suffers a heart attack, which proves fatal. Homer is freed from his work with the FBI without as much as a thank you, and is very depressed at the death. Homer visits Fat Tony's grave, throwing his disguise on his grave in a poetic apology. He is hit with a shovel from behind and gets knocked out.
Homer awakens to Fat Tony's voice. He then realizes that Fat Tony's nearly identical cousin, Fit Tony, is standing before him. The major difference between the two is Fit Tony wears a classic mobster tracksuit, and is in excellent condition. Fit Tony is going to whack Homer, but decides not to, because, he says, "my cousin lives on in your memories, and to extinguish you, would be to whack what still remains of him." Fit Tony takes his cousin's place running the Springfield mob. When the pressure gets to him, Fit Tony, who now dresses like Fat 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, The Simpsons way of making it like it never happened.
In this episode, it's stated that while Americans are already waking up from the New Year party and it's already January 1st, Samoa is still celebrating the New Year. However, due to its location towards International Date Line, Samoa is one of the first places in the world to celebrate the arrival of the New Year, way before the United States.