Haga Ineko stopped being a school teacher for she was about to get married. When she is suddenly dumped by her to-be husband, she loses her way, and starts drinking. One day, an old friend approaches with a job offer. She would be a substitute teacher in a class on which several teachers gave up without an apparent reason. She reluctantly takes the job only to discover that there are several brutal bullying cases inside the class. Even she eventually starts being anonymously threatened due to her opposing posture against such cases. She now has to identify and reprimand them while at the same time protecting weaker students. The word "Hagane" on the title is a nickname which comes from Haga (her surname) and Ne (from Ineko, her name), but is actually a pun with the actual word hagane (which means steel, denoting how she must be in order to resist as a teacher).