Blue Ribbon, Nightingale, the top assassin of Nation A, returns home to head the notorious Class Three to fulfill her late mentor’s last wish. With unmatched skills and sincere care, she tames her rebellious students and lifts the whole class’s grades. When her identity leaks, her students stand firmly by her side to pull her out of danger. In the end, she becomes their irreplaceable mentor, writing an endless legend on campus.