Plot
The story begins with a young boy named Dai remembering a story told to him by his adoptive grandfather, the monster magician Brass, about the defeat of the Demon King Hadlar by the hero Avan. After Hadlar's defeat, all monsters are freed from his evil will, and peace prevails for ten years. Many monsters move to the island of Dermline to live in peace. Dai, an orphan and the only human on the island, is raised by Brass alongside his best friend, the monster Gome, and dreams of becoming a hero.
After Dai befriends Leona, princess of the Kingdom of Papnica, and saves her from danger, Avan arrives on the island with his apprentice, the magician Popp, to become Dai's teacher at Leona's request. The training is interrupted by the return of Hadlar, resurrected by the Great Demon King Vearn to serve as commander of his army. Avan sacrifices himself to protect his disciples, and Dai temporarily drives Hadlar away by awakening a mysterious power within himself.
To honor Avan's final request, Dai, Popp, and Gome leave the island to defeat Hadlar and his master. During their journey, they are joined by the healer and martial artist Maam, Hyunckel—a former subordinate of Hadlar—and Leona. Their enemies include the Beast King Crocodine, the Mystic Legion commander Zaboara and his son Zamza, the elemental Flazzard, the elusive Mystvearn (Hyunckel's teacher of black magic and a greater servant of Vearn), the jester and personal assassin Killvearn, and Baran, a legendary warrior and Dai's father who sides with Vearn after his wife's death and his son's disappearance.
Crocodine and Baran eventually reform and while Crocodine joins Dai's side to defeat Vearn, Baran refuses to cooperate and decides to fight Vearn alone. After a failed first attempt to defeat Vearn, with Baran losing his life to protect Dai in the process, Vearn's true objective is revealed: Destroying the surface world with six powerful bombs called "Black Cores" spread across the world in order to have only the underworld remain.
Dai and his party regroup and successfully infiltrate Vearn's fortress in order to defeat him and stop the Black Cores, also helped by Hadlar, who also reforms and sacrifices himself for Dai and the others to proceed. With his subordinates defeated, Vearn reclaims his original body, which was in MystVearn's custody and fights the party alone with all his might. After a vicious battle, Dai triumphs and destroys Vearn for good. Dai returns and is celebrated, but Killvearn reappears, revealing himself as a robot controlled by his minion Piroro and intending to detonate the black cores. Dai sacrifices himself to stop Piroro and save humanity. In the epilogue, Leona notes that the gem on Dai's sword (which is connected with his soul) still glows, indicating that Dai is still alive and will one day return.
