Akuto Sai, a young orphan with aspirations of becoming a high priest to bring peace to the world, enrolls at Constant Magic Academy. However, during a routine aptitude test, his future is revealed to be that of the "Demon King," a tyrannical ruler destined to bring chaos and destruction. This revelation shocks the academy and its students, leading to widespread fear and suspicion toward Akuto. Despite his genuine desire to do good, his newfound reputation as the Demon King creates constant conflict and misunderstanding.

As Akuto navigates his new reality, he encounters several key characters who shape his journey. Junko Hattori, a skilled swordswoman and class representative, initially opposes him due to her duty to protect the academy but later becomes a loyal ally. Keena Soga, a carefree and enigmatic girl, forms a close bond with Akuto, often providing comic relief while hinting at deeper mysteries surrounding her identity. Korone, an android assigned to monitor Akuto, serves as both a guide and a source of exposition about the world's magical systems and political structures.

The narrative explores the tension between Akuto's destiny and his personal ideals. As he gains powerful abilities linked to his Demon King lineage, he faces opposition from various factions, including the government, religious organizations, and rival students. These groups view him as a threat and seek to eliminate him, often leading to intense battles and moral dilemmas. The story delves into themes of fate, free will, and the struggle to define one's identity in the face of societal expectations.

Throughout the series, Akuto uncovers secrets about the world's history, including the origins of magic and the true nature of the Demon King's role. The plot intertwines political intrigue, supernatural elements, and personal relationships, creating a layered narrative that balances action, humor, and introspection. The series concludes with a resolution that ties together the central conflicts while leaving room for interpretation regarding Akuto's ultimate path and the world's future.