In this alternate universe, mysterious Moderators secretly select certain individuals to become Kämpfer — powerful warriors forced to fight one another in brutal battles. Natsuru Senou is an 18-year-old senior at Seitetsu Gakuin High School, an ordinary student who suddenly discovers he has been chosen. As one of the rare male Kämpfer, his powers cause him to transform into a beautiful girl with long flowing blue hair, granting him devastating fire-based abilities and a special weapon.

Guided by a talking stuffed tiger named Harakiri Tora, Natsuru must now juggle his normal life as a male senior with his secret life as a female Kämpfer. He attends school in both forms under the same name, participates in deadly fights against other Kämpfer identified by their colored bracelets, and tries to manage the resulting chaos. This includes his longstanding crush on the energetic and popular Kaede Sakura, while dealing with rival Kämpfer such as the fiery Akane Mishima and the cool-headed Shizuku Sangō. Often reluctant and overwhelmed by his circumstances, Natsuru must adapt to his dual identity and the dangerous battles ahead if he wants to survive.