Mochizuki Kotaro is a mild-mannered, unmanly high school student. One day, he visits a library and meets the mysterious librarian Himiko, who recommends to him an ancient book about a heroic general from the Sengoku period 400 years ago. Kotaro notices that he has the same name and age as the general described in the book, and he experiences a flashback... Kotaro later mentions the book to his father, who tells him that he may be the descendent of a powerful samurai. At that moment, Kotaro receives a message from his childhood friend Ai telling him that their classmate Nakamura Tsuyoshi is in trouble. He rushes to the scene but doesn't have the guts to help, until another flashback transforms him into a samurai.
