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On Halloween night, Irwin, Billy, Mandy and Grim are trick-or-treating. Billy, Mandy and Grim decide to go home after having no success with Dracula (Irwin's Grandfather). Irwin realizes that his life is passing him by. Mindy (secretly turned into a witch by an evil marshmallow bunny named Bun Bun) fools Irwin into opening an Underworld portal releasing an army of evil candy warriors and Bun Bun, where they start terrorizing the neighborhood. Irwin gets attacked and contacts Hoss Delgado, who helps him with the battle. The warriors take down Hoss with soda-candy guns, which if Hoss gets sticky, he gets a rash. When they kill Skarr's winning purple pansies, he comes out of his garage with a robot, joining the battle. Soon it's an all out war, with no mercy, no rules, no nothing. Mindy again tricks Irwin, but this time, pretending she is being kidnapped by Bun Bun, taking her into the Underworld.
When the candy level gets to high, Hoss clicks his car unlock system, which a giant saw car (spoof of the Gotengo from Godzilla: Final Wars) comes drills out of the ground. They escape and start to plot their attack to the Underworld.
Soon they find a portal to the Underworld in the rain, which Hoss and Irwin jump in first (because he does not trust monsters and says to Skarr that he doesn't trust one eyed weirdos), but the portal turns off, because it is controlled by a diamond, which Mindy (in her witch form) smashes. The candy warriors try to sacrifice Hoss, by pushing him into hot coco, but they want to leave Irwin alive. This is because he has the powers of a mummy, which they need to keep the balance of the Underworld in place, and to become stronger warriors to eat the trick-or-treaters.
Mindy creates a potion which turns all the warriors into candy monsters, and all she needs now is Irwin's powers to create the candy warriors ultimate monster form. But Irwin refuses.
Soon Jeff The Spider, Fred Fredburger, and Skarr find out a way how to get to the Underworld by using Hoss's car to drill into the Underworld, and they rescue Irwin and Hoss.
But the army of candy monsters hasn't given up yet. They head to the city and attack. Irwin and Hoss get into an argument, leading into a battle, turning on each other. While the battle is going on, the monsters suck up Irwin's power (but not all of it) and transform into one giant candy monster. Jeff and Fred try to escape, but they fail. Soon Hoss and Irwin see the monster and start to fight it, beating it. Irwin discovers that Hoss is scared of monsters, because he was haunted as a kid.
Soon Bun Bun admits his history. He cut off Fred's tusks, he made Billy scared of spiders, he haunted Hoss's childhood, and turned Mindy into a witch. Skarr joins Bun Bun's team (but not really) and kicks Bun Bun into hot coco, melting the marshmallow bunny.
Soon Underfist gets an award for saving Halloween that year, but they hear something outside and investigate. It's soon revealed that there is an army of squid warriors who have found their way to the surface world that the team made while trying to escape from the Underworld.