Mairimashita! Iruma-kun centers on Iruma Suzuki, a 14-year-old boy 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. Despite being human, Iruma is thrust into the demon world, where he enrolls at Babyls Demon School. To protect his identity, Sullivan uses his influence to conceal Iruma’s humanity, presenting him as a demon with unique traits.

At Babyls, Iruma navigates a curriculum designed for demons, which includes mastering magic, combat, and demonic etiquette. His kind-hearted and adaptable nature allows him to form friendships with classmates, including the ambitious Asmodeus Alice and the mischievous Clara Valac. Iruma’s innate ability to avoid danger, honed from years of dodging his parents’ neglect, earns him the nickname "Misfit" and places him in the school’s most chaotic class. Despite his initial reluctance, Iruma gradually embraces his new life, often finding himself at the center of extraordinary events.

The narrative explores Iruma’s growth as he faces challenges unique to the demon world, such as summoning a familiar, participating in perilous exams, and competing in the annual Harvest Festival. His human qualities, such as compassion and resourcefulness, set him apart from his demon peers and often lead to unexpected victories. Over time, Iruma’s influence begins to reshape the demon world, challenging its traditional values of power and hierarchy. His relationships deepen, particularly with Asmodeus and Clara, who become his closest allies and confidants.

The series balances humor, action, and character development, with recurring themes of friendship, perseverance, and self-discovery. Iruma’s journey is marked by his gradual acceptance of his new identity and his ability to inspire those around him, even in a world where humans are considered prey. The story unfolds through episodic arcs, each highlighting Iruma’s adaptability and the evolving dynamics of his relationships within the demon world.