Bart and Homer are carving pumpkins and Bart slices a smile into Homer's pants. Homer chokes him, so Bart puts a flaming pumpkin on his head. Professor Frink then introduces the show, giving a warning about the content and telling those concerned to skip forward. He skips forward too far and says he has ruined the show. He points the remote at himself until he dies and turns to dust. This blows away to say "Treehouse of Horror XXI". The monster picks up the remote and mentions The Office. This leads to a parody of the opening, set in the monsters work place.
War and Pieces
Marge, worried about the effects of excessively violent video games, encourages Bart and Milhouse to give the wholesome, classic board games in the attic a shot. The best friends discover a board game called Satan's Path. Upon playing it, it causes all the games to come to life and alter reality as the two must beat all the games' challenges to restore everything to normal.
Marge, worried about Bart playing violent video games, encourages him and Milhouse to play the boardgames in the attic. They discover a game called Satan's Path and play it, causing all the board games to come to life in a different reality. They have to complete the games to win, which results in a game of Mouse Catch.
Master and Cadaver
Homer and Marge set sail on a second honeymoon, but is interrupted by a castaway, who they rescue. Roger was a chef on the Albatross who was knocked out after trying to prevent an attempted poisoning on the ship. Convinced that Roger poisoned the passengers and plotting to murder them after seeing a dead shark with a recently made pie pan in its mouth, Homer and Marge take matters into their own hands as they seemingly killed Roger and knock his body overboard. However, finding the Albatross, they realized Roger was telling the truth. Roger appears again and Homer then kills Roger, and the surviving bankers, which shocks Marge who eats the poisoned pie.
It would all turn out to be Maggie's imagination.
Daniel Radcliffe, the main star of the Harry Potter franchise, guest starred in the third segment which parodies the Twilight novels and movies. Radcliffe said that he is a fan of the show and guest starring is "a huge honour". Hugh Laurie guest starred in a segment that parodies Dead Calm.