"Welcome to Demon School, Iruma-kun" centers on Suzuki Iruma, 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. To ensure Iruma’s safety and integration into demon society, Sullivan enrolls him in Babyls Demon School, a prestigious institution for young demons. Despite being human, Iruma must navigate this dangerous and unfamiliar environment while concealing his true identity.

At Babyls, Iruma quickly becomes entangled in the school’s hierarchy and traditions. He is placed in the misfit class, where he befriends two demons: Alice Asmodeus, a fiery and competitive demon who becomes fiercely loyal to Iruma, and Clara Valac, a cheerful and eccentric demon with a knack for mischief. Together, they form a close-knit trio, often finding themselves in chaotic but endearing situations. Iruma’s kind-hearted nature and willingness to help others earn him the admiration of his peers, despite his initial struggles to adapt to the demon world.

The series explores Iruma’s growth as he faces various challenges, including academic trials, magical competitions, and social dynamics unique to demon society. A recurring element is the "Evil Cycle," a phenomenon where demons experience heightened aggression and power. Iruma, being human, is unaffected by this but must find creative ways to navigate its effects on those around him. His resourcefulness and compassion often lead to unexpected victories, earning him recognition and respect.

Sullivan’s influence as a powerful demon and the school’s chairman provides Iruma with protection, but it also attracts attention from other high-ranking demons, including the scheming Kalego, Iruma’s homeroom teacher. Kalego is initially suspicious of Iruma but gradually acknowledges his potential. Meanwhile, Iruma’s rise in status and his growing list of achievements, such as becoming the class president and forming the Misfit Class’s student council, further complicate his life as he balances his human identity with his demonic surroundings.

The narrative also delves into the broader demon world, introducing characters like Ameri Azazel, the student council president and daughter of a powerful demon family. Ameri develops a fascination with Iruma, particularly after discovering his love for human manga, which is forbidden in the Netherworld. Their interactions add a layer of intrigue and humor to the story.

Throughout the series, Iruma’s journey is marked by themes of friendship, perseverance, and self-discovery. His ability to inspire those around him, combined with his unique perspective as a human in a demon world, drives the plot forward. The series balances comedic moments with heartfelt character development, creating a dynamic and engaging narrative that explores the complexities of demon society through Iruma’s experiences.