Gruñón, a red ogre dwelling on the secluded Monster Island, inhabits a realm feared by humans yet populated by monsters equally terrified of humanity. His grumpy nature and difficulty in social interactions render him unpopular among fellow monsters, stemming from deep-seated hatred toward both humans and ogres. This fuels his solitary existence, broken only by his bond with Garzo, a blue ogre. Their connection originates in shared orphanhood after losing parents in the Human vs. Monster war centuries prior.

Gruñón's isolation shatters when a human child, Kotake—alternately called Mushroom or Champiñón—arrives on the island. Despite initial reluctance and ingrained animosity, Gruñón becomes Kotake's primary caretaker. This responsibility sparks unexpected emotional growth, fostering newfound tenderness and care through his interactions with the child and sustained relationship with Garzo.

A pivotal shift occurs when Kotake wishes to return to his human family. Demonstrating evolved compassion, Gruñón escorts the child toward the human village. There, villagers violently attack Gruñón upon sight, tearing Kotake from him. The loss plunges Gruñón into profound depression. Witnessing this despair, Garzo makes a consequential decision to secure Gruñón's future, triggering an irreversible separation between the ogres. These events cement Gruñón's journey from bitter isolation to connection and sacrifice.