At the movies watching Fuzzy Bunny, Alvin convinces his brothers to sneak into another theatre with him to watch the horror movie Hideous Harold Xl and they all end up terrified. That night during a storm the brothers fear Hideous Harold will come, and Simon devises a series of traps built from their toys. Dave ends up locked outside while changing a fuse that was damaged in the storm and the Chipmunks are woken up by him rapping on the door to be let in. When the Chipmunks see the silhouette of him wearing a fisherman’s hat they mistake him for Hideous Harold, and they think Dave not being in his room and the phone not working is Hideous Harold’s doing. They discover they were wrong when they manage to capture Dave in a snare trap. Alvin admits it was his fault for theatre-hopping, but Dave says all three Chipmunks are at fault for going along with Alvin's suggestion. The following day the Chipmunks are stuck inside doing chores as punishment. Theodore is glad he is over his fears of Hideous Harold, but Alvin boasts he was never scared at all, only to hide behind the couch in fear when an advertisement for the next Hideous Harold movie comes up on the TV.
- The title of the episode and the horror movie Hideous Harold are based on the movie A Nightmare on Elm Street.
- Goof: When Alvin clenched his right hand while saying "We'll have to make Hideous Harold tell us where Dave is," he has five fingers instead of the usual four.
- Part of the Alvin and the Chipmunks: Nightmare on Seville Street VHS.
- Part of the Trick or Treason and Halloween Collection DVDs.