An ordinary high school boy named Mushiki Kuga encounters a mortally wounded woman, Saika Kuozaki, who is secretly the most powerful mage in the world. In her final moments, she transfers her body and immense magical abilities to Mushiki, entrusting him with the duty of protecting the planet from recurring annihilation events that threaten existence every three hundred hours.

Mushiki awakens in Saika’s body and must assume her identity as the headmistress of a secret academy for mages known as the Garden of the Void. He struggles to control the unfamiliar powers he has inherited while hiding the truth from classmates, teachers, and even his own younger sister, Ruri Fuyajo, who is a talented student at the academy. The only person aware of the switch is Kuroe Karasuma, Saika’s loyal attendant, who helps Mushiki navigate his new life. A unique complication arises because Mushiki begins to revert to his original male form whenever he is not in close contact with a woman, forcing him to rely on unexpected kisses to maintain the disguise.

The world is defended by mages who battle mysterious destructive entities known as Annihilation Factors, with the Garden of the Void serving as a key training ground. Mushiki must master Saika’s power while investigating the circumstances of her apparent death and dealing with rival magic institutions. Notable characters include the popular magic video streamer Kurara Tokishima, who suspects the truth about Mushiki’s identity, and powerful knights such as Elluka Freera and Anviette Svarner who serve under Saika. The story follows Mushiki’s efforts to live up to the legacy of the world’s strongest witch while keeping his secret, growing into his new role, and uncovering a larger conspiracy that threatens everything Saika worked to protect. An anime television adaptation was announced in late 2025.