Plot
Twelve year old Marshall is a horror movie fanatic while his best friend Gilbert does not share his interests, having walked out of a horror movie called Warthead that they were both seeing. Marshall's divorced mom is seeing a man named Ted that Marshall has trouble adjusting to and his mom wants him to give Ted a chance. Marshall and Gilbert see some people putting a dead body in their creepy neighbor Mr. Kubot's house.
One day, during a field trip to a museum, Marshall and Gilbert are paired up with a new girl in town named Amy, who just moved from a town called Rockport and doesn't like where she is now, and she and Marshall bond over their love of horror movies, much to Gilbert's dismay. While at the museum, they see the Egyptian exhibit and learn a story about a mummy couple being separated and that one of the mummies that was supposed to be there mysteriously disappeared.
Amy learns about the dead body in Mr. Kubot's house and believes that it's the missing mummy and they all sneak inside the house to find the mummy, and Gilbert finds the scarophagus and runs away when he finds the mummy, which comes to life and chases the kids around and gives Marshall back his flashlight that he dropped while running. Then Kubat and his goons come back and the kids sneak out.
The kids then discover the mummy following them and Marshall has to sneak him into his room while Ted and his mom are watching a movie and he teaches him how to eat pizza and hot sauce, and names him Harold as they start to bond.
Meanwhile, while Marshall is at school, Harold starts wandering around the house and gets scared by technology and runs out of the house and starts wandering around town, where he finds the museum where his girlfriend’s sarcophagus is. Marshall, Amy, and Gilbert decide to make a mummy movie with Harold as a school project, but they discover that Harold escaped while they were at school and they find him at the museum and they discover that an amulet that he's wearing brought him to life.
They take him to Marshall and Gilbert's monsterologist friend Buzzy for help on what to do with him and she finds hieroglyphics on the amulet and decides to translate them. Meanwhile, the kids start to work on their movie and Ted gives them costumes for it, and Amy tells Marshall about her divorced dad marrying her stepdad that she didn't like at first, but eventually accepted him and she tells Marshall to do the same with Ted, then they start to work on the movie.
Buzzy successfully translates the amulet which says that both mummies must be in their sarcophaguses by the third day that he came back to life, or he'll turn to dust. They soon discover that Kubot's men took Harold's sarcophagus out of his house and Kubot finds Gilbert's inhaler in his basement, and he discovers that the kids stole him, and he and his men confront Harold and the kids, but they manage to escape, and Kubot sees that Harold is alive and could be worth a lot of money.
Harold and the kids go to their school's Halloween carnival to find Buzzy to get her to drive them to an abandoned ice cream factory, where Harold's sarcophagus is, and Harold starts dancing at the carnival. Meanwhile, Kubot and his men arrive at the carnival and start chasing Gilbert and Harold through the haunted house and Kubot takes the amulet off Harold, turning him back to normal and they take off with him, and Buzzy and the kids follow them to the ice cream factory.
Kubot and his men deliver Harold to Ravensworth, an art collector and they show her the amulet bringing Harold to life, which impresses her. The kids arrive into the factory, but Gilbert bails on them not wanting to get killed, but later decides to face his fears after talking to an owl.
Marshall, Amy, and Buzzy sneak in and Buzzy gets chased by Kubat's men while dressed as a werewolf, and she and Amy get captured and Marshall warns them that Harold will turn to dust if Harold doesn't go back into his coffin, but Kubat won't listen, then Gilbert appears and gets the amulet back and throws it back on Harold and he breaks free of his chains and starts attacking the henchmen and Kubot and Ravensworth get knocked out with a trap that Buzzy set. Gilbert admits that he came back because he was really scared about losing his friends and says that he's not scared anymore.
Buzzy and the kids then break into the museum where Harold reunites with his girlfriend and the kids says goodbye to him as he goes back into his coffin, but not without Marshall giving him an empty bottle of hot sauce to remember him by.
Meanwhile, the news reports that Harold was delivered to the museum and that Kubat and his goons were arrested and Marshall invites Ted to be in his next movie, having starting to accept him and tells his mom that they should be together if they're happy together. Amy also decides to stay in town and she, Marshall, and Gilbert all go see Warthead again.
