When Lisa becomes frightened at the Krustyland Halloween Horror Night, Homer is forced to take down his Everscream Terrors decorations. Halloween pop-up employees seek vengeance on Homer for causing them to lose their jobs, and rob the family house while Homer and Lisa take shelter in the attic. Homer and Lisa light the stored holiday decorations to attract attention and stand up to the intruders. Meanwhile, Marge takes Bart out for trick-or-treating only to discover that it is too late.
The day before Halloween, Homer completely decorated the family house, calling it "Everscream Terrors", but Homer stored the plastic skeletons too close to the furnace, so he chose to take its remainings back to the Halloween pop-up store. On their way to the store, Lisa and Bart commented about how excited they are about the upcoming Krustyland Halloween Horror.
At the shop, Apu (who chose a Halloween joke name: A-Boo) gets furious at three lazy pop-up employees who are sleeping on their job, telling them to go back to work. But one of them gave Homer a deal, where if he buy him one "Senor Skeleton", a dancing mexican skeleton, he would give Homer three of them for free. Homer accepts it and talks about the deal to Apu, who promply fired the bad workers. They then promised revenge on Homer, saying he will be really sorry for getting them fired.
Later, Homer takes the kids to the Halloween Horror, but Lisa becomes terrified and asked her father to go back home, but because the tickets were too expensive and he and Bart were enjoying the park, Homer thought she should only think that all of that isn't real and stay on the park. This didn't help at all, and Lisa ends up getting lost in the park and traumatized to a point where the employees are forced to shut down the park.
At school, Lisa's trauma made her scared even from the simplest Halloween figures, such as paper zombies and bats. She then hides inside a locker, only to be rescued later by Marge. Back home, Marge realizes Lisa's problem and tells Homer they should shut down Everscream Terrors (as Lisa is scared to the point of getting help from Tailee, a plastic animal tail), making both Homer and Bart upset. In order to reconcile with Bart, Marge takes him and Maggie to the best Block Party in Springfield, leaving Lisa and Homer alone.
At home, Homer is visited by the three ex-pop-up employees, where they started playing mind games with him. Homer then realizes they are being stalked by the criminals while trying to keep Lisa calm during Halloween. Meanwhile at the block party, Marge discovers that it is for residents only, as last year's party was wrecked by visitors. Bart gets even more upset because he has "missed Halloween".
Back at 742 Evergreen Terrace, Homer tries to lock the house, not realizing that the three employees are already inside. He tries to take Lisa to the Flanders' House, but she runs back inside to get Tailee, the only thing that kept her calm during Halloween. Homer rushes to save her, only for them to get face to face with the home invaders. Homer and Lisa then rush to hide at the attic while the ex-employees are looking for them.
But matters aren't that great to Marge, Bart and Maggie's Halloween. As it is too late, all the kids are asleep, and it's time for the grown-up Halloween, (which is when grown ups dress up in costumes to get drunk), so Marge plans to take the kids back home. At the Simpsons' House, Homer calms down Lisa, and they choose to use the Everscream Terror decorations as a help signal, but they accidentally activate the Senor Skeletons, giving away their location to the marauders.
Homer climbs onto the roof to light up the remaining fourth of July fireworks, but the wind extinguishes the matches. Lisa then remembers that Tailee is made of polyester and will burn pretty easily, so Homer burns Tailee to light the fireworks to attract people's attention, and use the lights to ask for help. The home invaders are then arrested almost immediately and Homer builds Everscream Terrace back so the town can enjoy it.
During the credits, Maggie finds Tailee. It grows back from being burnt and she becomes obsessed with it.
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