The Grim Adventures of Billy and Mandy provides three different modes of play, as well as a tutorial and extras. The tutorial is relatively short, but it does show you how to use all of the controls. The one thing it doesn’t do is wait until you master a skill before going on to the next one. You had better pay attention or you’ll miss how to do something and either have to figure it out in battle or go through the tutorial again.
The first mode you can choose after the tutorial is Story Mode. When you first start the game, you’re given the choice to play as Billy, Mandy, Grim, or Irwin. You’ll unlock more characters in the Mission Mode. Story Mode is pretty short. Your goal is to recover Grim’s mojo from a mysterious enemy. You’ll play through the different areas as whichever character you chose, sometimes working with a computer-controlled partner, fighting the various enemies that you’re familiar with from the cartoon including Eris, Nergal, Hoss Delgado, and many more, searching each of them to find out who stole the mojo.
The next mode you can choose is VS Mode, and it’s exactly as it sounds. VS Mode allows you to play up to four players in a melee battle. To quote Nergal, “The best things in life must be taken by force.” If you haven’t played through Mission Mode to unlock the extra arenas and weapon sets though, you’ll have limited choices, so go to Mission Mode and take things by force!
Mission Mode consists of five tiers with nine challenges in each tier. You start out with access to tier 1 only. To gain access to each of the consecutive tiers, you have to complete five of the eight available missions. To get the ninth mission and complete each tier, you have to beat all eight available missions. After pretty much each mission, you’ll unlock something new, including concept art and alternate clothing. At the end of each tier, you’ll unlock a new character to play as. To view all the unlockables that you just got, such as character bios, weapon sets, and concept art, you go to the Extras area.