The Bob Next Door
The Simpsons Season 21 Episode 22 (05/16/10)
When the Simpsons neighbors move out, a nice man named Walt moves in next door. Walt sounds exactly like Sideshow Bob, yet no one believes Bart. Marge takes Bart to see Sideshow Bob in prison to prove the killer is still behind bars. Content to believe Walt is not Bob, he joins him for a trip to a baseball game.
Bob escapes from prison and reveals that he is in fact the real Walt, Bob's cell mate and that Sideshow Bob switches faces with him Face/Off style. Walt knows Bob's plan to kill Bart in five corners and takes the Simpsons family with him there. Walt arrives just in time to stop Bob from killing Bart... with the help of Wiggum and the other four state jurisdictions.