When Billy destroys Grim's favorite childhood toy, Fancy Jake, Grim feels devastated and wishes he had never lost the limbo game that made him become Billy and Mandy's friend. Then, a lawyer appears saying that although the game is legally binding while all parties remain on Earth, it is not if Grim leaves Earth. In consequence, Grim follows the lawyer's advice, opens a portal, and leaves. Then, with Grim travelling aboard his spaceship, the lawyer takes off his disguise and introduces himself as Morg, the Martian Grim Reaper, who is also Emperor of Mars and commands an army of martian, brain-eating zombies. Then, so Grim can get the respect he deserves, Morg convinces Grim to conquer Earth, which they do easily while enslaving and turning all the people into zombies. Next for the reapers is to use the ultimate weapon in mind control, the Braincaster, which, if powered by a dumb mind, will allow them to control all the zombies forever. Meanwhile, Billy, Mandy and Irwin, who followed Grim to Mars hidden in his bottomless trunk, look for a way to save Earth. Then, when the kids are captured, and Billy is strapped to the Braincaster, all hope seems lost; however, at the last minute Billy talks to Grim about friendship, and Grim has a change of heart and decides to help Mandy. In the end, Billy's stupid brain overpowers the Braincaster, returning all the people to normal and foiling Morg's plan, who disappears into outer space.