Somebody is sending Bart threating letters, written in blood. It is actually Sideshow Bob, who is soon released from prison. Upon discovering that it was Sideshow Bob who sent him the letters, Bart and his family join the Witness Relocation Program.
The family move to Terror Lake under the new name "The Thompsons", living in a houseboat. However, Sideshow Bob straps himself to the underside of the car and follows them to their new home.
In the middle of the night, Bob cuts the boat loose from the dock, ties up the Simpson family and chases after Bart, cornering him. Stalling for time, Bart asks Sideshow Bob to sing the entire score from the HMS Pinafore as a last request.
Just as Bob finishes, the boat enters Springfield and crashes into some rocks. Bob is arrested and sent back to prison.