Gil is a mage who earned the title of "Great Sage" in this life, having been a teacher in Japan in his previous one. Unable to refuse a request from a close friend, he takes up a post as a "substitute teacher" named Geek at a magic academy while continuing his research. However, what awaits him is a crumbling institution rife with problems a place that has lost all semblance of a proper educational facility. From noble students abusing their status to look down on teachers to insidious bullying and corruption involving the faculty, Geek ruthlessly intervenes to confront the academy's dark underbelly that threatens his students. Thus begins a teaching fantasy starring the ultimate substitute teacher!