The character known as "Champiñón" is the human child Kotake (コタケ). Kotake accidentally enters Monster Island, a domain inhabited by creatures fearful of humans due to past conflict, causing widespread panic. He finds care with Naki, a red ogre harboring dislike for humans and other monsters, and Gunjo, Naki's sole friend, a blue ogre. Living alongside them, Kotake forms a bond. His presence helps the emotionally withdrawn Naki gradually open up and experience tenderness.

When Kotake begins longing for his human family, Naki resolves to return him. Villagers violently attack Naki upon witnessing the delivery, forcibly tearing him away from Kotake. This traumatic event plunges Naki into depression. Gunjo subsequently acts to aid his friend, resulting in a permanent separation between the two ogres. Kotake acts as the catalyst for the central relationship development between Naki and Gunjo, and his departure directly triggers the film's emotional climax.