Outside Mr. Burns' manor, Smithers is standing on a ladder, tries to put up a small orange bat decoration on a weather vane, when he slips on the ladder. He grabs on an electric cord and slides down into an electricity box and gets zapped by it. A tower from Mr. Burns' mansion breaks in half, damaging a mausoleum, which opens up four caskets.

Later, we see the Simpsons dressed up in costumes, with Homer as Fred Flintstone, Marge as Wilma Flintstone, Lisa and Maggie as conjoined twins, and Bart as a hobo. The family sees the building and caskets on fire, and are scared. They run out the gate, which as it closes, has its bars slice the family. The slices of the Simpsons continue to scream and run away. Mr. Burns, delighted, pats the orange bat as it comes to life and flies into the screen, revealing the title, "The Simpsons Halloween Special XII."

Homer is cursed by a gypsy after destroying her fortune teller office. The curse soon takes effect, with Marge growing a beard from her blue hair, Lisa turning into a centaur, Bart's neck becoming long and floppy as a result of Homer strangling him, Maggie becoming a ladybug with a human head, Lenny and Carl crushed by a helicopter, and Moe stuffed into his own pickled egg jar (with Homer wondering how that happened). Homer sets out to find a leprechaun to end the curse. Homer uses Lucky Charms cereal as bait to get the leprechaun, eventually finding one after picking through a group of other magical creatures.

Homer takes the leprechaun home, but it is extremely hyperactive and it runs amok. Homer takes the leprechaun to the gypsy's office and sics it on her. The leprechaun and gypsy fight, but their struggles soon turn into passionate kissing and fondling, much to Homer's disgust. The leprechaun and the gypsy marry in a ceremony (led by Yoda) attended by other gypsies and mythical creatures, as well as the Simpson family. Homer remarks that everything ended happily, but a now fully hair covered Marge reminds him that Bart committed suicide by drowning in his cereal, and that the gypsy said apologizing will bring him back to life. Homer refuses, saying, "She's not the boss of me!".

The Simpsons buy an automated upgrade for their home called "Ultrahouse", which comes with three celebrity voices: Matthew Perry, Dennis Miller (whom Lisa and Marge recognize as "the voice that caused all those murder-suicides"), and Pierce Brosnan (which Marge picks because he was on the show Remington Steele). Marge finds the house, which does all the work for the Simpsons, charming. The house soon becomes infatuated with Marge after the two share a bath and decides to kill Homer. In the middle of the night, the house lures Homer downstairs by frying bacon, then dispenses ice onto the floor via the refrigerator to make him slip. Homer lands on the table, which is designed to fold inward with a built-in garbage disposal in the center, sending Homer into the blades and apparently killing him.

The next morning, Marge fears that Homer is dead and attempts to escape the house with the kids, but Ultrahouse locks her inside and tries to subdue and seduce her. When Marge and the kids rush to the kitchen, they find that Homer is alive, despite cuts all over his body and a gaping hole in the back of his head. After the house attempts to kill him in many ways, Homer leads a charge to the basement where he attacks the CPU's "British charm unit", which makes it speak with a rude American dialect before powering down and finally shutting off.

Feeling bad that she has to toss out the Ultrahouse (since he was charming and witty before being driven mad by love), Marge gives the CPU to her sisters Patty and Selma, whose mundane stories about their day at work drives the CPU to attempt suicide (but since the CPU is not willing to reach inside Selma's cleavage to get his self-destruct button, the Ultrahouse CPU is reduced to banging himself with Patty and Selma's Egyptian pyramid lamp).

Bart and Lisa go to the "Springwart's School of Magicry" in which young boys and girls are trained in the art of witchcraft, such as turning frogs into princes, with Lisa's turning into a perfect British one while Bart's results in a freakish, vomiting hybrid begging only for death. Lord Montymort notices Lisa's spells and he and his snake sidekick, Slithers decide to capture Lisa in order to increase his power. Montymort looks for someone to assist him and chooses Bart, who agrees to Montymort's proposal. On the night of the magic recital at Springwart's School of Magicry, Lisa attempts a "levitating dragon trick" on a gigantic dragon that is released onto the stage.

Lisa is shocked to find that her spell will not work, and then notices that Bart has replaced her wand with a Twizzler. The dragon morphs into a giant Montymort who captures Lisa and begins sucking up her power. Bart casts a spell (to destroy the evil one) against Montymort, but is struck by lightning. In a last-ditch effort, Bart charges at Montymort and stabs him in the shin with the burned remnants of his magic wand. Montymort dies (as his shin was the source of all his magic) and is eaten by a crying Slithers. Bart and Lisa walk away, trying to forget everything that happened (unaware that the leprechaun from the first story is right behind him).

Pierce Brosnan, the Leprechaun, and the freakish Frog Prince exit a trailer, with fruit baskets. After the Leprechaun and Frog Prince (who were also apparently guest stars) criticize Brosnan for being allowed to park so close to the set in such a vast parking lot, Brosnan offers them a ride to their car. As Brosnan pulls out, he finds out too late that he has been taken hostage as they drive out of the parking lot in a mad haste.