Senou Natsuru is your normal, everyday high school student. However, one morning he wakes up and finds himself turned into a beautiful girl. A stuffed tiger Harakiri Tora tells him then, that he has been chosen to be a Kämpfer (German for "fighter"), who has to fight other Kämpfer with either guns, swords or magic.
Natsuru has been granted great strength and fighting abilities on top of the power to use magic, but finds out that only girls can be a Kämpfer. Left without a choice, he is forced to turn into a girl again and again to fight other Kämpfer — to survive.