Premise
One year after meeting Marshall and Gilbert and reuniting Harold the Mummy with his girlfriend, Amy is preparing for her father, Pop's Halloween-themed wedding to his fiancé, Carl. Meanwhile, a worker at a museum accidentally revives an evil mummy Sobek, who was once Harold's best friend and they were both in love with Rose (whom Amy named Harold's girlfriend) and Rose chose Harold and Sobek wants to seek revenge.
Buzzy and the kids arrive at Rockport and meet up with her dad and Carl, and get settled into their hotel and go get their rings and jewelry store guy sees a picture of Sobek's amulet and tells them that Sobek may be after Harold and Rose and that the amulet brought him to life.
They go back to the museum and reunite with Harold and Rose by using sunlight on the amulet to bring them to life. Buzzy uses replicas of the mummies to put in their sarcophaguses so they can sneak them out of the museum.
Back at the hotel, Marshall orders pizza for him and Harold and Marshall updates him on his mom and her boyfriend Ted, who is still going strong and that Gilbert and Amy are making videos for the school paper together, although Marshall admits he is jealous about the amount of time they always spend together.
While Buzzy and the kids are preparing to decorate for the wedding, Amy finds out that Sobek hypnotized a museum worker and they stole an Egyptian scepter and are on their way to Rockport to seek revenge on Harold and Rose, who sneak off to the Halloween carnival. Meanwhile, Sobek and his goons arrive at the museum and discover the fake mummies and decide to go find the real ones at the festival.
Buzzy and the kids find the mummies dancing at the festival where they encounter Sobek about to attack them with the scepter and they escape in Buzzy's car, but Sobek and his goon wind up with the wedding rings and learn from the jewelry store owner where Amy's dad's wedding is.
Back at the hotel, Amy discovers the rings are missing, which upsets her, but she decides not to worry about it and just enjoy herself at the party and Marshall tells Gilbert and Amy to keep an eye on the mummies at the party. Meanwhile, Sobek arrives at the party and causes a mess at the party and Sobek takes Rose's amulet and kidnaps her and attacks Harold when she tries to save her.
Amy gets upset when the party gets ruined and locks herself in her room confesses to her dad and Carl that she lost the rings and Marshall gets upset and accuses Gilbert and Amy for not listening to him about protecting Rose and Harold comforts him.
Amy apologizes to her dad for ruining things and he and Carl tell her they do not need a perfect wedding or rings to get married. Marshall confesses to being jealous of Gilbert and Amy spending a lot of time together and they apologize to him for leaving him out and they discover a small Egyptian exhibit in a nearby warehouse where Sobek is.
They arrive at the warehouse and devise the plan to get the amulet and revive Rose. They attack his goon and tie him up and revive Rose, but Sobek finds them and uses the scepter to revive more mummies to attack Harold, Rose and the kids, but Gilbert climbs onto a moon and drops it down on the floor and crushes the mummies. But Sobek decides to kill both Harold and Rose if he cannot be with her and the kids use shields to deflect the scepters attack to destroy the jewel on the scepter and the amulet, defeating Sobek and breaking the spell put on his goon, whose real name is Larry and he gives Amy back the wedding rings and attends the wedding as Buzzy's date.
Amy gives her dads their rings back and the wedding goes on as planned and everyone celebrates, including the mummies. Then Buzzy and the kids take Harold and Rose back to their sarcophaguses and say goodbye.
The movie ends with them going out to eat and they start to think about next Halloween, even though Marshall wants to recover from the current Halloween season.
