"Treehouse of Horror XXX" is a Halloween-themed episode of the long-running American animated sitcom The Simpsons, It is the fourth episode of season thirty-one, the thirtieth Treehouse of Horror episode, the thirty-first Halloween-themed episode of The Simpsons (including the 2015 episode "Halloween of Horror") and the 666th episode of the series overall. It first aired on the Fox network in the United States on October 20, 2019.
The series is based around the Simpson family, made up of nuclear power plant worker Homer Simpson, his homemaker wife Marge, their ten-year-old son Bart, their eight-year-old daughter Lisa and their baby daughter Maggie. There are also a great many other recurring characters in the series who live in the small town of Springfield along with the Simpsons.
In common with many other Halloween-themed episodes of The Simpsons, "Treehouse of Horror XXX" is made up of four short self-contained stories, an extended opening sequence before the episode's opening credits and three main segments. The titles of the episode's three main segments are "Danger Things", "Heaven Swipes Right" and "When Hairy Met Slimy". None of the events of any of the stories in the episode are considered to be canon as far as the series' main continuity is concerned.
The opening sequence depicts baby Maggie Simpson as a destructive and murderous child of Satan.
In "Danger Things", which is set in the 190s, Bart's friend Milhouse disappears into another dimension. The segment is a parody of the science fiction web television series Stranger Things that has been airing on Netflix since 2016.
The segment "Heaven Swipes Right" focuses on the relationship between Homer and Marge. Homer dies and his spirit then possesses the bodies of several other characters. The segment parodies the 1978 movie Heaven Can Wait, which is itself a remake of the 1941 film Here Comes Mr. Jordan and was remade in 2002 as Down to Earth. The segment's title also references the dating app Tinder.
In "When Hairy Met Slimy", Marge's sister Selma falls in love with the alien Kang. The segment is a parody of the 2017 dark fantasy romantic film The Shape of Water. Its title references the 1989 romantic comedy film When Harry Met Sally.