新上任的小学女教员芳贺稻子（吉濑美智子 饰）现年35岁，拥有10年的教龄，可谓经验丰富，人称“钢之女”。任职于爱河小学的稻子被委托教导四年三班。这个看似普通的班级却先后让三任班主任辞职离去，而辞职的原因却不明。表面上天真无邪的四年三班的小朋友们，背地里到底做了什么，让大人们摇头离去。稻子带着疑惑开始了和这群小朋友及他们背后的家长的战斗。

原来在和平的表现下，是学生家长对学校的各种无理要求和傲慢态度，而小朋友们也学着大人的模样，越来越趋于成人化。这群恶魔小孩妄图掌控班级，而钢之女的出现，让这一情况开始发生扭转，和她共同战斗的还有男教员盐田涉（要润 饰）。

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). (d-addicts.com)

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