Due to a janitor's mistake, an unsuspecting custodian is turned into Janemba, a chubby, yellow demon causing chaos in the afterlife. Since his heroic death during the battle against Cell, Goku Son has been taking part in posthumous martial arts tournaments. Janemba's disturbances eventually draw the attention of Goku and his fellow fighter Pikkon, prompting them to search for a way to defeat the monstrous enemy.
After their clash, Goku witnesses Janemba undergo another transformation: the demon sheds its excess weight and changes color, becoming a sword-wielding opponent far more powerful than any Goku has faced before. Alongside his equally deceased rival, Vegeta, Goku must stop the menacing Janemba from destroying the very fabric of the universe.