The 30th Treehouse of Horror features a demon Maggie, a mission to rescue Milhouse from another dimension, dead-Homer's spirit trying on some new bodies for size and Selma finally finding love in an unlikely place - the alien in the basement.
In a parody of the movie The Omen, at the Springfield General Hospital, we find out that the third baby Marge had was actually a boy, but Homer didn't want another boy, as Bart is bad enough. Dr. Hibbert offers that he adopts Maggie, who actually has demon-like powers, which she uses to choke Hibbert with his stethoscope and burn him.
We see Maggie use her powers to spin around stuffed animals heads, make them vomit up their stuffing, and make Kearney and Jimbo hang themselves.
Ned sees Maggie's devilish abilities and attempts to kill her at a abandoned church. Homer and Marge try to stop him, but Ned shows them that Maggie 'bears the mark of the beast'. He shows, under her hair, a mark that looks like Mickey Mouse. This is the wrong mark, however, and the real mark is the numbers '666', the number of the Devil.
Before Ned can kill Maggie, she rises in the air, and breaks open the roof of the church. Homer states that Maggie still better than another boy, and all three them get giant wooden stakes through their hearts. The classic Treehouse of Horror logo appears, with the staked Ned, Marge, and Homer appearing as the Roman Numerals (XXX).
In parody of the Netfilx show Stranger Things, where Milhouse is featured as Will, Bart as Mike, Nelson as Dustin and Lisa as Eleven.
The story is set in the 1980s, where after a night of playing the Atari 2600 game E.T. the Extra-Terrestrial, Milhouse falls victim to kidnapping by a Demogorgon. His worried father Kirk begins going insane by destroying the wall as well as creating a Christmas light a means of communication (not unlike what Joyce Byers used to communicate with her son, Will, in Stranger Things). Milhouse in the Over Under (A parody of 'The Upside down') decides to contact the Simpson household with Lisa picking up. After finding out Lisa does not returns his feelings he hangs up. Lisa heads to Starcourt Mall (infested by rats that change the sign into Rats court Mall) to find Bart and Nelson playing at the arcade. She tells them that Milhouse is still alive and in another dimension. They decide to get help from Jonathan Frink in order to travel to the Over Under. Lisa (bald now) finds Milhouse in the other dimension and try to escape till they find an alternate version of Springfield full of Demogorgons (such as Moe and Comic Book Guy). Lisa and Milhouse surrounded by Demogorgons so Lisa uses her psychic powers to get rid of the creatures. They are finally by Homer, who is working for Mr. Burn's secret government program to find monster. Sadly, they are trapped in the in the dimension so The Simpsons decide to live in the Over Under.
Homer is happy that there are no Ned Flanders in the Over Under until a Demogorgon version of Flanders appears, causing Homer to burn the Demogorgon.
Heaven Swipes Right
In parody of Heaven Can Wait, while watching a football game Homer accidentally chokes on a hot dog and dies. However, it is discovered that Homer should not be dead yet but cannot return to his body as it is too decomposed. He is placed in the body of handsome athletic man. Marge is delighted to Homer again but quickly ruins his new body by eating junk food. He next takes the body of Gary Chalmers and Bart uses him to torment his principal, Seymour Skinner. However, when he takes a look at his pay check he dies of shock. After taking the body of a monkey who looks like Mr. Teeny, he ends up taking Moe Szyslak's body. In a twist, it is revealed that Moe has taken the body of Maggie much to Marge's disgrace. This is also a parody of the 90s show "quantumn leap".
When Hairy Met Slimy
In a parody of The Shape of Water, featuring Selma as Elisa and Kang as The Amphibian Man. Selma falls for an alien and helps him escape captivity with the help of Homer. Pursued, they meet up with his sister Kodos at the top of Mt. Springfield. Selma is shot and killed by the pursuing soldiers and Kang destroys them with The Infinity Gauntlet. Kang resurrects Selma by telling her that she is pregnant. Patty then arrives to tell Selma to not leave Earth but falls in love with his Kodos, when they both admit their sexual preferences to one another. They all honeymoon together with Patty and Selma nagging their spouses.
The episode finishes with a collage of clips from previous Treehouse of Horror episodes, followed by every other episode, announcing this one is episode 666.