Summary: A kid picks at his dinner, teased by his sister about being afraid of a monster in his closet. Mom is wimpy and Dad is fretful about the kid's masculinity. With a knife spinning dangerously in the garbage disposal, a monster breaks through the kitchen window and pulls Dad's head towards the blade.

It's just a movie, exciting Marshall but terrorizing Gilbert who prefers fare such as The Sound of Music. Halloween is approaching but they probably won't bother with costumes. Marshall finds out that Gilbert is too afraid to collect newspaper subscription money from the creepy Mr. Kubat, so they approach the door. Threatened by a growling dog, they run to Marshall's house where Marshall begrudgingly greets his mother and her boyfriend Ted. His room is a horror chamber and it's confirmed that he's bent about his parents' divorce.

At Roosevelt Junior High School, Amy reports that Kubat, who worked in a New York museum at one time, was found dead -- her mother is the real estate agent. The three kids go to the house and Gilbert sees some "coffins" in the basement -- sarcophagi with a mummy hand sticking out of one. He loses his glasses and runs. They retrieve the house key secretly from Amy's mother and return. A mummy attacks, it seems at first, but its lumbering is awkward, and after a bathroom break it's clear that it means no harm. It even returns Gilbert's glasses. Marshall's beeping watch scares it. It touches Amy's hair: "Must be my new conditioner. It attracts dead guys." They leave, worrying about experimentation and harm if they break their pact of secrecy and treating the creature like a dog: "Stay. Good boy."

At school they discuss the impossibility of trusting any adult authorities. "It's ours. We're going to keep it." They also define the mummy as a monster: "Mummies don't eat people.... That's zombies." Mummies "reek." The three visit Bruce at his novelty store and, pretending to be researching for a school project, have him read more lore. Bruce's dusty book reports that on the "annual festival of the dead" -- interpreted here as Halloween -- the mummy's spirit may be freed, so long as there's a full moon, but he must return to his coffin by midnight.

A small brat announces that a big yellow truck took Kubat's belongings and Amy's mother is showing the empty house to prospective buyers. Supposedly the IRS confiscated the belongings for back taxes.

The mummy is running loose. It creates confusion at the Dairy Freeze drive-thru, then sees a bandaged guy being wheeled into the Mapleton Hospital and so follows. His wrapping gets caught in the door and he starts unravelling. He gets into the buzzer at a reception desk, hurts himself with a clipboard, is embarrassed when he realizes he has been unwrapping, and is mistaken for a burn victim who self-treated. The doctors find no pulse or heartbeat and try zapping him, so he runs away. The kids see him and decide to bring him to Marshall's house, maybe saying he's a foreign exchange student. He blends into Marshall's room. They name him Harold and douse him with Hi Karate because of the stench. Marshall opens up emotionally to the mummy.

Some thugs have the empty coffin. Meanwhile, as Marshall sleeps, the mummy plays with a slingshot, encounters a dog, rides a skateboard, and, seeing a pyramid symbol on a bus, lumbers towards a "mysteries of the pyramids" museum exhibit. The kids track him down and decide he can read the hieroglyphs, that he was a celibate high priest secretly in love with the Queen, and that he instinctively knows he needs to be back in his sarcophagus. Cops show up so they all hide.

Amy is worried that Marshall has become too emotionally attached to the mummy. When they decode the littler brat's "yellow truck" blabbing, they head for an old ice cream factory with the mummy in disco clothes for a disguise. As they approach, Marshall finds the binoculars useless. Amy retorts, "When you get your surveillance equipment out of a happy meal, what do you expect?" Kubat, the man at the door with the dog originally, turns out to be the leader of four goons. One spots the kids and mummy and reports to Kubat.

The heroes don costumes: Marshall a skull mask, Amy as Princess Leia, and Gilbert as "Mrs. Howell from Gilligan's Island?" -- no, Dennis Rodman with a boa. They go to a haunted house party at Bruce's establishment. Amy and Marshall connect about the kid's angst regarding his potential stepfather. The goon enter after the mummy, who bows down to a Cleopatra and creates confusion since Gilbert's principal, Mr. Hammer, is also in mummy costume. Marshall and Amy try to explain everything to Bruce, but he doesn't believe them until Cleopatra unwraps Harold's face and a chaos of screaming breaks out with the goons in pursuit. The living make a getaway through the funhouse and in Bruce's hearse, but Harold is captured.

At the factory, Gilbert chickens out and stays in the vehicle. Armed with grisly novelty items, the others tie up the look-out with fake intestines and enter the factory to save Harold, who is chained to some piping until Marshall crowbars him loose. But all are captured and brought to Kubat, who explains that he had hoped to live off pilfered museum pieces throughout retirement until the IRS started investigating him, so he faked his own death and hopes to make a bundle on Harold. Gilbert drives through the wall in the nick of time and Harold whips the gun out of Kubat's hand. They take Harold back to the museum where he says good-bye to the briefly re-animated queen mummy and returns to his sarcophagus.

We see Marshall packing away his monster paraphernalia. He asks his mother if she truly loves Ted, and since she says she does he says they should be married. Now, in front of Gilbert, Marshall is self-conscious about Amy and grooming himself. Gilbert wonders if he has sustained a "severe head injury." At the door, Amy and Marshall are awkward at first, but quickly dissolve into a palsy exchange of insults. So all is well and the three go off to see another movie starring a horror creature: "He's a monster. He eats people." "That is such a cultural stereotype!"