It's Halloween and the siblings Wern and Reynold, new to town, are going trick-or-treat through the neighborhood in order to make some new friends. The player can choose between both of them, while the other will be kidnapped by a monster neighbor. The quest, as expected, consists on rescuing your brother/sister.
Costume Quest is a RPG with turn-based combat. The game takes you door by door asking for candy and fighting against the monsters that you find in some of the houses with the power of your costume. There is around a dozen of them and they have the power to transform the wearer into vampires, robots, the statue of liberty and many other creatures. The combats plays through synchronized button pushing much like in the Mario RPGs. Combat abilities are enhanced by acquiring experience points through a typical leveling system. Additionally, "combat stamps", which is the name used in the game for combat enhancements, can be obtained in the side quests (like finding collectibles or helping the characters of the neighborhood), with candy as the currency to buy these.