The Grim Adventures of Billy and Mandy is a game based after the Cartoon Network series of the same name. Grim's chest was tampered with and Mojo Balls were released into the universe. These Mojo Balls infect everyone they touch with supernatural rage. So Grim, Billy and Mandy have to go out and retrieve them all to prevent the universe to fall into eternal chaos.
The game's Story mode is a side-scrolling platformer/brawler, which alternates between small platforming sections and stopping to have the player battle a certain amount of enemies, all of which have to be defeated before the level can move on. To accomplish this the player has kick and punch attacks as well as an air attack and a special attack which can be launched once the power bar is full. It is filled by defeating enemies. There is also a double jump and a butt bounce action, which help to get through the platforming sections.
At the end of each character's story is a boss, whose defeat awards the player with one Mojo Ball. The Mojo Balls are needed to unlock the extra characters. Initially only Billy is available, then Mandy, and last Grim. To unlock Mandy Billy's Story must be completed and a certain amount of Mojo Balls collected. Most of the Mojo Balls come from Mission mode, which plays essentially the same, but in very short levels, many of which have time limits and play like mini-games.