High Quality Content by WIKIPEDIA articles! "Treehouse of Horror XXI" is the fourth episode of The Simpsons` twenty-second season. It first aired on the Fox network in the United States on November 7, 2010. This is the 21st Treehouse of Horror episode, and, like the other "Treehouse of Horror" episodes, consisted of three self-contained segments: In "War and Pieces", Bart and Milhouse discover a real-life board game that they must win to return home; in "Master and Cadaver", Marge and Homer go on a honeymoon on a sailboat, and rescue a mysterious castaway named Roger; and in "Tweenlight", Lisa falls in love with a vampire named Edmund.