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 is still better than another boy, and all three of them get giant spear-like weapons stabbed through their hearts, revealing the title: TREEHOUSE OF HORROR XXX.

In the '80s, at Bart's treehouse, Bart is playing E.T., with Martin, Milhouse and Nelson. After getting a game over, the kids split up to go back home on their bikes, when Milhouse gets attacked.

Luann and Kirk gets worried as he hasn't come back at home, and Kirk starts busting the wall trying to find him. The police finds his bike near the woods, while Kirk places letters on the wall to see if he's in another dimension and he can comunicate with them.

Since Milhouse is gone, Luann confesses she's having an affair, and Disco Stu comes down the stairs revealing it's him. Milhouse sends an SOS with the letters but due to the problems they're having they send him and his lights "in his room".

At the Simpsons home, Lisa gets a call from Milhouse, but once she says she doesn't like him as a boyfriend, he hangs up. Lisa goes to the Starcourt Mall, which sign gets invaded by rats to become the Ratscourt Mall. The kids are playing at the arcade and she informs them of Milhouse being trapped in the other dimension, and he calls for them from the videogame Bart was playing.

They go to Professor Frink's lab to find a way in, and it's the sensory deprevation tank. In the Over Under, Lisa finds Milhouse, but they're spotted by the Demogorgons that bites his left leg off when he doesn't stop talking.

They try to escape the Domegorgons in town, while other monsters devastates the city, and Lisa uses her psychic powers but they're trapped till Homer saves them with a flame thrower, saying it was Mr. Burns opening the portal to find monsters for a governament secret project, to find monsters. Homer informs them they're all trapped in the dimension so they sit down at the couch before finding out there's a Ned version of the Domegorgon and burning him.

At the Springfield Atoms Stadium, Homer, Moe, Lenny and Carl are watching the Springfield Atoms vs. Shelbyville Shelbyvillians match, and when the crowd starts shouting choke, a hotdog ends up choking Homer to death.

Arriving in Heaven, he meets St. Greeter, who welcomes him with the pun on his name, but Homer doesn't forgive the pun so he gets damned to Hell. Homer finds out God sold Heaven to Google, before discovering he died before his time, but can't be sent back to Earth in his body due to the decay.

He could decide he could live the life of a man that was going to die that day, so Homer chooses a football player's body. He goes to Marge and she loves the new body, but he ruins it in one night. Bart and Marge convinces him to try Superintendent Chalmers as the next body.

At school, Homer makes Skinner give Bart all As and twist his nipple till its deep purple, and then goes to his house just to see he makes very little money and changes again, but Marge has had enough. Homer has to settle in for one body and stop changing, so he chooses the one of the man that loves her as much as he does, Moe, however Moe finds himself in Maggie's body and informs her he's thirsty.

At the Power Plant, Selma enters a maximum security door to sneak a smoke, and finds herself in a laboratory, where she meets Kang and they fall in love, but Mr. Burns wants to disect him, so they flee, to go to a distant galaxy, with Homer's much obliged help.

They bring Kang in a waste container full of rats, but Burns finds out once Kang eats Smithers, so he follows them but his driver is shot so they flee to Mt. Springfield. Thinking they're safe, Kodos arrives with their ship but a shooting takes place and Selma is hit in the stomach.

Kang then pulls out Thanos' infinity gauntlet to eliminate all military people, and when Burns survives, he hits him with a thin can. Kang then heals Selma and tells her she's pregnant.

Patty arrives, protesting they're too different, being of different zodiac signs, but Kang points out he's from Sagitarius. Patty begs her not to go but falls in love with Kodos. They fly to space, and find them self on the honey moon planet, in the cool season, where it's 4.000 F.