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 realizes that his life is passing him by. Mindy fools Irwin into opening an Underworld portal releasing an army of evil candy warriors and Bun Bun, who 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 is an all out war, with no mercy and no rules. Mindy tricks Irwin again, 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. 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, but the portal turns off, because it is controlled by a diamond, which Mindy smashes. The candy warriors try to sacrifice Hoss, by pushing him into hot cocoa, 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. 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 down to it. They rescue Irwin and Hoss. But the army of candy monsters has not 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 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 child. 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 and kicks Bun Bun into hot cocoa, melting the marshmallow bunny. Soon Underfist gets an award for saving Halloween, but they hear something outside and investigate.
- The Grim Adventures of Billy and Mandy: Billy and Mandy's Jacked-Up Halloween
- The Grim Adventures of Billy and Mandy: Grim or Gregory?