After saving the world from the destructive might of Majin Buu, Gokuu Son and his allies look forward to a well-earned rest. However, unbeknownst to them, their battle was recorded and observed by Gomah, an evil being who assumes the title of King in the Demon Realm, filling the void left behind with the death of former ruler Dabura.

At Gomah's side is Degesu, the younger brother of Higashi no Kaioshin, a being who lords over the universe. Jealous of his brother's power, Degesu works alongside Gomah to steal Earth's Dragon Balls and preemptively subdue any potential future enemies. Calling upon the power of the legendary dragon Shenron, Gomah wishes for Gokuu and his friends to be reverted into children, intending to rob them of the limitless power that defeated Buu. With Earth's greatest warriors shrunken down, Gomah abducts Dende, the planet's acting god.

But Gokuu refuses to be set back by his new body, working to master his younger form to fight as proficiently as he could as an adult. Joined by Kaioshin and Glorio—a mysterious but seemingly rebellious demon—Gokuu pursues Gomah to the Demon Realm in order to rescue Dende and restore his friends to their natural ages.