 After receiving numerous death threats in the mail , Bart becomes paranoid . It is revealed that the writer is his enemy , Sideshow Bob , who is incarcerated in Springfield State Prison . The next day , Sideshow Bob 's parole hearing is held ; the parole board is easily convinced that Sideshow Bob is no longer a threat . When the Simpson family goes to the cinema , Sideshow Bob sits in front of them and acts obnoxious . The Simpsons then realize that it was him who sent the letters , and Marge angrily tells him to stay away from Bart. 
