Young Dai lives on Dermline Island, where he dreams of being a Hero. One day, a Hero and his companions come to the island. Dai is excited at first, but their true intent is to steal his best friend, Gomechan, the mythical Golden Metal Slime.

Dai must escort Princess Leona to the Earth Cave for a royal ritual. But danger lurks where they least expect it.

A dark power affects Dai's monster friends, driving them berserk. Brass tries to get Dai to leave. As they argue, a Hero trainer arrives to inform Dai that the Dark Lord has returned. To save those he cares about, Dai undergoes Hero training.

Avan turned into a dragon and attacks Dai. Dai fights back to no avail. To defeat Avan's flames requires Wave Slash. As Dai clears his objective, someone enters the island. Who should appear before Avan but the worst possible being...!

Avan is overpowered by Hadlar, who is now stronger than ever. To protect his students and the world, Avan gathers the last of his strength and faces Hadlar. Dai can do nothing but watch. Finally, the showdown between Avan and Hadlar is concluded.

Dai leaves the island with Popp to defeat the Dark King. They’re saved from monsters in the Dark Forest by a girl named Maam. But the ferocious Beast King Crocodine appears!