Twenty years after the hero's party defeated the Demon King, ending the war between humans and demons, the world has entered an age of adventurers who plunder dungeons and slay monsters for riches and glory. In a dungeon known as the Steam Oven, a brash C-rank adventurer named Tina is gravely wounded by a monster. She is rescued by a massive, lion-headed figure who appears to be another monster—but is in fact Garon, the former Beast King, once the most feared of the Demon King's six generals. To the shock of everyone who believes he died in the final battle, Garon is alive and has become a healer. He saves Tina's life but imposes a blood contract of absolute obedience in exchange. Under this contract, Tina must accompany him on his journey.

Garon's mission is twofold: to heal wounded monsters and restore the damaged dungeons that regulate the flow of ley lines—underground currents of magical energy that sustain the land itself—and to find a cure for an incurable illness afflicting the Demon King's daughter, whom he has sworn to save. As they travel, Tina, a materialistic and fame-seeking adventurer, gradually learns that the dungeons are not mere treasure troves but delicate ecosystems essential to the world's balance, and that monsters are not mere obstacles but living beings with their own suffering. Garon treats all life—human, demon, and monster alike—driven by his belief that death must not become a curse and that as long as one lives, a curse can turn into a blessing.

The story unfolds across several major arcs. In the Steam Oven arc, Garon and Tina treat a Ruby Dragon whose child was killed by adventurers, restoring the dragon's will to live and the dungeon's vitality. In the Moon Mirror Castle arc, Garon reunites with his former subordinate Crow, who has been guarding a crumbling dungeon for two decades. To save the dungeon, Crow must sacrifice himself by entering a mirror world forever. Garon must reconcile his past as a commander with his present as a healer. In the Fist Saint Gareus arc, Garon's comrade Mikage is captured by Gareus, a former member of the hero's party who now suffers from a rotting disease known as the Earth's Poison. Gareus, driven by human supremacist beliefs, seeks to destroy all demons and their dungeons. Garon confronts him, not to kill but to cure, using medical knowledge and strategy to neutralize Gareus's power and force him to live as a patient instead of dying as a warrior.

Tina grows from a self-centered adventurer into a bridge between humans and demons, earning her B-rank status and defending Garon's ideals even against other humans. The central conflict culminates in a confrontation with Kuzeli, a demon who once vied with Garon for the title of Beast King. Kuzeli, who has spent twenty years nursing hatred for humanity and clinging to pure-blood demon supremacy, cannot accept Garon's alliance with a human girl. Their battle is not merely physical but ideological—coexistence versus purity, healing versus destruction. Garon ultimately defeats Kuzeli but chooses to forgive and save him rather than destroy him, embodying the theme that the strongest power is not to kill but to heal.

The story concludes with Kuzeli's redemption, the restoration of the world's ley lines, and a hopeful vision of coexistence between humans, demons, and monsters. Garon's journey redefines the meaning of "Beast King" from a warlord to a gentle king who uses his strength to protect life.