The game is essentially a "save the princess game". You and a merry band of teammates have to storm a castle of whom your enemy has kidnapped the princess. They've been feeding her cake though, making her fat, and thus making it harder for you to escape with her.

Many have considered Fat Princess to be Sony's Castle Crashers, and the gameplay supports up to 32 players instead of just 4. Hopefully we won't have to wait much longer for it to get released.