"Kaibutsu-kun" centers on a young boy named Kaibutsu-kun, who hails from the Monster Kingdom, a parallel world inhabited by supernatural beings. He is sent to the human world as part of a mission to study human behavior and culture. Accompanied by his loyal companions, including the bat-like Wikking and the mischievous Duck, Kaibutsu-kun navigates the complexities of human society while maintaining his monstrous identity. His interactions with humans, particularly a kind-hearted girl named Hitomi, form the core of the narrative, blending humor, fantasy, and lighthearted drama.

The series explores Kaibutsu-kun's attempts to balance his monstrous nature with the expectations of human society. His unique abilities, such as transforming into a giant monster, often lead to chaotic yet comedic situations. The Monster Kingdom's ruler, King Enma, occasionally intervenes, adding layers of conflict and intrigue as Kaibutsu-kun strives to fulfill his mission without disrupting the human world. The dynamic between Kaibutsu-kun and his companions provides both comedic relief and emotional depth, as they grapple with their roles in the human world.

Throughout the series, Kaibutsu-kun encounters various challenges, from misunderstandings with humans to rivalries with other monsters. His relationship with Hitomi evolves, highlighting themes of friendship, acceptance, and cultural exchange. The episodic structure of the series allows for a mix of standalone adventures and overarching plotlines, ensuring a balance between humor and character development. The narrative maintains a lighthearted tone while subtly addressing the challenges of coexistence between different worlds and species.