On Halloween night, Irwin, Billy, Mandy, and Grim set out to enjoy a night of trick-or-treating, each embracing their own unique costume and personality. Billy becomes a witch, Mandy and Grim take on roles inspired by the Spanish Inquisition, humorously burning witches and vampires as part of their playful antics. Irwin is dressed as Dracula, the king of vampires, adding a touch of classic horror to the night. Despite their efforts, the group struggles to find success, especially when Irwin encounters Dracula, who rudely dismisses them, handing out only pennies, much to their disappointment.
As the evening progresses, Billy, Mandy, and Grim decide to head home, feeling unfulfilled, while Irwin remains eager to make the most of the night. Irwin soon has a moment of reflection, realizing that his busy, unfulfilling life is slipping away. Meanwhile, a girl named Mindy (secretly transformed into a witch by an evil marshmallow bunny named Mr. Bun Bun) tricks Irwin into opening an underworld portal. This act unleashes an army of evil candy warriors, which quickly begin to terrorize the neighborhood with their chaotic and destructive behavior.
The candy warriors are aggressive and relentless, attacking residents and leaving chaos in their wake. Irwin quickly contacts Hoss Delgado, a tough and resourceful hero, to help handle the situation. Hoss battles fiercely, but the candy warriors manage to take him down using soda-candy guns; these weapons pose a special hazard for Hoss, as getting sticky with them could cause a rash. Meanwhile, when Skarr, a villain with a penchant for purple pansies, comes out of his garage accompanied by a robot, the conflict escalates into a full-scale war involving no mercy or rules.
Mindy continues her deception, once again tricking Irwin by pretending she is being kidnapped by Mr. Bun Bun and taking him into the underworld. The situation worsens as the candy warriors’ strength increases, leading to the deployment of a giant saw car—emerging from the ground when Hoss activates his car’s unlocking system—providing a dramatic escape route for the heroes. They then plan their next move to confront the threat of the underworld.
The heroes eventually find a portal in the rain that leads into the underworld, but their efforts are thwarted when Mindy destroys the diamond that powers the portal, causing it to shut off. As the candy warriors attempt to sacrifice Hoss by pushing him into hot cocoa, their true intentions reveal themselves: they want to keep Irwin alive because he possesses the unique powers of a mummy—being half-mummy through his mother—and need him to maintain the balance of the underworld. Without Irwin, the army of candy monsters would lose their strength and be vulnerable.
Mindy then creates a potion to transform all the candy warriors into monstrous creatures, intending to harness Irwin’s powers for their ultimate form. However, Irwin refuses to comply with her plan. Just then, Jeff the Spider, Fred Fredburger, and Skarr discover a way into the underworld using Hoss’s car, successfully rescuing Irwin and Hoss from the chaos. Despite their rescue, the candy monsters continue their assault, heading to the city and unleashing their terror.
During the final confrontation, Irwin and Hoss get into a heated argument that quickly turns into a fierce battle, leading to unintended clashes and misunderstandings. The candy monsters manage to absorb Irwin’s powers, transforming into a colossal candy creature. Jeff and Fred attempt to escape, but their efforts are thwarted. Recognizing the danger, Irwin and Hoss join forces to fight the giant candy monster, eventually managing to defeat it.
In a revealing moment, Hoss admits that he’s afraid of monsters because of childhood hauntings, and he apologizes to Irwin for his earlier attitude, understanding that not all monsters are bad. Both Mindy and Hoss experience growth as they realize their mistakes and express regret for their actions. Meanwhile, Mr. Bun Bun and Skarr make an appearance; Mr. Bun Bun confesses that he was behind Mindy’s transformation into a witch, using a Witch Worm to manipulate her and reach Irwin. He also admits to having haunted Hoss’s childhood and causing fear in Billy, as part of his sinister plans.
As Mr. Bun Bun prepares to kill Mindy and Underfist, Skarr, secretly working for him, betrays Mr. Bun Bun by kicking him into hot cocoa, melting the marshmallow bunny in the process. With the danger temporarily averted, Grim, Billy, and Mandy (who has now become president) commend Underfist for their heroism. However, their victory is short-lived when they hear strange noises outside, revealing that a new threat has arrived: an army of squid warriors that has managed to infiltrate the surface world.
The movie ends with the team preparing for what promises to be an even greater battle, showing that the adventures—and the chaos—of Halloween are far from over.
