"Welcome to Demon School, Iruma-kun" centers on Iruma Suzuki, a 14-year-old human boy who is sold to a demon by his neglectful parents. The demon, Sullivan, is a high-ranking figure in the Netherworld and adopts Iruma as his grandson. Sullivan enrolls Iruma in Babyls Demon School, a prestigious institution for young demons, where Iruma must navigate a world entirely foreign to him while hiding his human identity.

At Babyls, Iruma quickly becomes the center of attention due to Sullivan’s influence and his own unintentional feats. Despite his initial fear and lack of confidence, Iruma’s kind-hearted nature and resourcefulness earn him the respect and friendship of his classmates. His closest companions include Alice Asmodeus, a fiercely loyal and powerful demon who becomes his self-proclaimed rival and protector, and Clara Valac, a mischievous and energetic demon who brings chaos and humor to their dynamic. Together, they form a trio that faces various challenges within the school.

The series explores Iruma’s growth as he adapts to demon society, where strength, ambition, and cunning are highly valued. Through a series of trials, competitions, and school events, Iruma gradually gains confidence and begins to embrace his new life. His unique human traits, such as empathy and adaptability, often give him an edge in situations where brute force or traditional demonic methods fail. Over time, Iruma’s actions and achievements lead to him being recognized as a rising star within the school, even as he continues to conceal his true identity.

The narrative also delves into the hierarchical and competitive nature of demon society, with recurring themes of ambition, loyalty, and self-discovery. Iruma’s relationships with his classmates and teachers evolve as he proves himself capable of overcoming challenges, often through unconventional means. The series balances humor, action, and heartfelt moments, creating a vibrant and engaging portrayal of Iruma’s journey in the Netherworld.

Key supporting characters include Sullivan, who dotes on Iruma and provides guidance; Opera, Sullivan’s stoic and capable assistant; and various teachers and students who contribute to the school’s dynamic environment. The series also introduces overarching mysteries, such as the origins of the Demon King and the significance of Iruma’s growing reputation, which add depth to the narrative. Through its blend of comedy, fantasy, and character-driven storytelling, "Welcome to Demon School, Iruma-kun" presents a compelling tale of an ordinary boy navigating an extraordinary world.