Twenty years after being summoned to another world and serving as a hero before retiring from the military, Washio now works as a low-ranking biology teacher at the Royal Mune Innium Academy in the kingdom of Medium. Still single and in his mid‑forties, he is routinely mocked by students and colleagues alike, earning the insult “bald vulture.” His stagnant life is upended when a gorgeous girl with tentacles named Claudia transfers into his class and announces, “I’m the monster girl you once saved!” She is a kraken‑type monster who evolved from a small octopus Washio rescued years earlier. Soon other former monster pupils—including a spider‑type girl, a dragon‑type girl, and a selkie—arrive at the academy, all having mutated into humanoid “monster girls” and all harboring romantic feelings for the man who once helped them.

The story follows Washio’s sudden, bewildering popularity as these beautiful former students vie for his attention while he struggles to maintain professional distance. Their presence draws scrutiny from the school administration and uncovers a conspiracy involving a mysterious figure named Julius, who seeks to exploit the monster girls for his own ends. An early major arc sees Washio wrongfully accused of causing a monster girl’s rampage and forced to leave the school; the monster girls then investigate the truth, uncovering a plot that threatens their safety. Later arcs explore Washio’s suppressed past as a powerful hero, the trauma that led him to retreat into a quiet teaching life, and the growing danger from those who wish to weaponize the monster girls. The narrative balances romantic comedy with darker undertones, as Washio gradually confronts his own history and begins to reclaim his former strength.