A 33-year-old single office worker named Yamada suddenly finds himself transported to another world. In this realm, where many species live for hundreds of years, a person in their thirties is considered no more than a small, fragile elementary school child. Stripped of his adult status, Yamada is enrolled in a local school where he becomes classmates with a group of fantastical children, including a young demon king, a succubus, a golem, and a manticore. The story follows Yamada’s daily life as he navigates the absurdity of his new situation, struggling to reconcile his identity as a mature salaryman with the reality that everyone around him sees him as a cute and helpless little boy. The comedy arises from this role reversal, as Yamada is doted on by his demon king classmate, praised for performing simple subtraction, and generally treated with overwhelming, childlike affection despite his age. Over the course of the narrative, he learns about this strange world and its inhabitants through a series of episodic school-based adventures and cultural misunderstandings. The narrative is structured around the growing and often hilarious relationship between the world-weary Yamada and his powerful but childlike demon king peer, as well as his interactions with other monstrous children. Notable storylines include Yamada being "adopted" by the demon king who becomes his caretaker, his struggles to fit in with a curriculum designed for creatures of myth, and the various ways his adult knowledge proves unexpectedly useless or oddly relevant in a world where physical age is no indicator of experience or wisdom.