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 realizes he, along with others, have lost their wallets to a pickpocket. Meanwhile, Fat Tony meets with his uncle with his mob 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 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 and says to him that if Homer can have confidence, he can, too, so Wiggum sets out to prove Fat Tony's innocence and the man that framed him. Tony testifies and says he would never pickpocket because his father, Fatso Antonio, died from his heart being pickpocketed. Chief Wiggum later visits Fat Tony his jail and 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 Chief Wiggum, Fat Tony hears one of the men in his group mention how easy he is to frame and the new boss is revealed to be Johnny Tightlips, and Tightlips says Paxil gave him the confidence to betray him, and assures his new role as boss with Frankie the Squealer, Johnny Kissass, Sonny Go-Along, and Joey Can't-Read-The-Room. Homer comes in, wondering what sauce his pasta had on it and the police close in. However, Homer is shot in the rear and Wiggum ends up sucking it 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 Fat Tony then enjoy a toast at Giuseppe's.