Costume Quest

The game takes place on Halloween. Fraternal twin siblings Reynold and Wren are new in their neighborhood and are asked by their mom to make new friends in the neighborhood. The player at this point chooses which sibling to play as, marking them as "in charge". The two dress in their costumes, with the non-playable character dressing as a piece of candy corn and the playable character dressing as a robot. A monster sees the non-playable sibling, and kidnaps him or her. The player character must now save their sibling in time and be home before curfew. The player character trick-or-treats at houses, tracking the monsters and their sibling from the neighborhood, then to a mall, and then to a country town. Along the way, they meet fellow trick-or-treaters Everett and Lucy, who join their party. Eventually the player character confronts the leader of the monsters, Dorsilla, who after defeating her reveals that she is actually working for a much more powerful monster named Cadaverous Big Bones. By defeating him, the player character rescues their sibling and returns home, banishing the monsters back to their home world.

"Grubbins on Ice"

"Grubbins on Ice" takes place after the original game in the Winter season. Everett and Lucy are searching for any evidence of the monsters that was left behind. Lucy finds a piece of evidence, which opens a portal that she is sucked into. Everett then takes Reynold and Wren with him to find and save Lucy, who has been taken captive by the monsters on the other side of the portal, which leads to the monster home world of Repugia. Similar to the original game, the player must trick-or-treat at various houses to eventually reach Cadaverous Big Bones, defeat him once more, and rescue Lucy. At the end of "Grubbins on Ice", the main characters escape from Repugia into a nexus of portals, which leads directly into the plot of Costume Quest 2.