Natsuru Senō wakes up one day finding himself turned into a girl and that the stuffed tiger, Harakiri Tora, he was given by the love of his life can now speak. Natsuru learns that he has been selected to be a Kämpfer and is destined to fight other Kämpfer. Harakiri explains that each person selected to become a Kämpfer is given a bracelet that transforms them whenever they are near other Kämpfer and that Kämpfers must be females. Natsuru must learn to keep transformations a secret while trying to survive attacks from other Kämpfer and not to mention handling the fact that his high school love is actually falling for his female transformation.