Like many high school students, Kotaro Mochizuki (Haruma Miura) doesn’t find life easy. His mother nags him to study harder and get into a good university. High achieving scholars and popular athletes outshine him at school. Mochizuki laments that he is just another loser. But Mochizuki does have a heart. He can’t bear to see his pathologically bumbling and shy classmate Tsuyoshi Nakamura (Yuu Shirota) playing into the hands of the bullies. Being clumsy and timid himself, however, Mochizuki can’t do much to defend Nakamura.

That is until he discovers that he has magical samurai powers. After reading a book that describes historic, heroic feats of a warrior samurai who shares his name, Mochizuki finds that in a pinch, a samurai spirit and strength overtake his body, elevating him into a fearless fighter who spews forth pearls of wisdom of the "bushido" or "code of the samurai."

The comedy drama is based on serious issues in the school yard such as inequality, discrimination, harassment and bullying, but presents them in a light-hearted manner through exaggerated characters and grandiose actions. The fantastical transformation of Mochizuki from awkward student to valiant samurai is both humorous and cathartic and will thrill audiences, especially those who have been secretly yearning for their chance for payback.

望月小太郎（三浦春马饰），是典型的害羞怕事型的男生。对什么事都「装没看见」，没自自信，不中用的高中生。有一天他在图书馆内发现了一本古书，内容是有关一位400年前勇敢武士的事迹。小太郎发现那位武士和自己是同名同姓，还是同月同日生，同是17岁。忽然，小太郎的脑海中闪现了自己身穿武士服，手拿武士刀冲向敌阵的画面。小太郎从父亲信二（岸谷五朗饰）的口中得知自己的祖先原是武士时，大为意外，莫非自己忽然能变成古代武士是因为祖先是武士？

一天，小太郎收到好友爱（杏饰）的求救电话，说被不良学生挑衅。小太郎赶到后，本欲救出爱，可是却没勇气与不良少年交手。在千钧一发之际，小太郎忽然化身成为武士……到底之后会发生什么事情？

从那次之后，每当遇到困境，小太郎就会变身为武士，解决在学校内外发生的各种问题。