Simpsons S21E04. The longest running animated series in America, the Simpsons is back with the latest episode. The latest is the Simpsons Season 21 Episode 4 Treehouse of Horror XX. In this episode, the 20th anniversary of THE SIMPSONS annual “Treehouse of Horror” Halloween special, three chilling tales of Halloween horror unwind. First, Lisa becomes livid with Miss Hoover, and in a series of homages to classic Hitchcock sets out for deadly revenge. Next, Krusty’s latest fast-food sandwich transforms news anchor Kent Brockman into an enraged zombie and twenty-eight days later, cannibal zombies have overrun Springfield. See Bart and the gang and find out how this episode went. Watch the Simpsons Season 21 Episode 4 Treehouse of Horror XX.