The Fat Blue Line is the third episode of Season 31, originally the Season 30 finale in production order.
During the San Castellaneta street festival, much of the town realizes their wallets have been pickpocketed. After the state investigator puts the wrong man in jail, Chief Wiggum sets out to clear his name, catch the real criminal, and show he's still up for police work.
The Simpsons go to the San Castellaneta street festival and Homer Simpson realizes he, along with others, have lost their wallets to a pickpocket. Meanwhile, Fit-Fat Tony meets with his uncle with his mafia crew. Chief Wiggum is on to crack the case until Lenora Carter takes it and steals Wiggum's job. Homer is selected by Carter to bait the suspect with a wallet in his back pocket as he has the biggest rear end in town. The bait is taken and Fit-Fat Tony is arrested but claims to be innocent, as he thought the warehouse with all the stolen wallets was an Asian pop fusion restaurant. He is taken to jail, but assures his innocence.
Meanwhile, Wiggum, depressed, talks to Marge Simpson and says to him that if Homer can have confidence, he can, too, so Wiggum sets out to prove Fit-Fat Tony's innocence and the man that framed him when he watches an old video of Fit-Fat Tony’s testimony that he would never pickpocket because his father, Fatso Antonio, died from his heart being pickpocketed before it was gonna be used for his upcoming heart surgery. Wiggum later visits Fit-Fat Tony in his jail cell and Fit-Fat Tony proves his innocence by showing Wiggum something he always does during the time everybody pickpocketed; Singing show tunes alone. Wiggum then promises him he will get him out of jail if he can prove his innocence.
With the help of Wiggum, Fit-Fat Tony, upon arrival at Luigi’s with the police and given a wire to summon them once he finds the real culprit, hears Frankie the Squealer in his mafia mention how easy he is to frame and after Fit-Fat Tony forces Frankie to confess by threatening him, Frankie reveals that the new boss and that the wallet pickpocket is revealed to be Johnny Tightlips, and as Fit-Fat Tony angrily confronts Johnny about this, Johnny then admitted that Paxil (His medication) gave him the confidence to betray him, and assures his new role as boss with Frankie, Johnny Kissass, Sonny Go-Along, and Joey Can't-Read-The-Room. Homer, meanwhile, was eating dinner with his family at the same time and place when he comes into the same back dining room, wondering what sauce his pasta had on it and the police, upon being called in by Fit-Fat Tony’s wire, close in and arrest the mafia. However, Homer is accidentally shot in the rear by Johnny when Johnny meant to shoot Fit-Fat Tony and Wiggum ends up sucking the bullet out, saving Homer's life.
In the end, Wiggum gets his job back, but with no kind words from Carter, but instead Marge, telling him she believes in him. He, Homer, and Fit-Fat Tony then enjoy a toast at Giuseppe's.