Arisa, a 16-year-old high school girl, accidentally has an extreme fighting suit that her brother invented implanted in her body. It's a slightly embarrassing suit that makes her emit an H voice when she transforms. Moreover, from that day on, Arisa is forced against her will to fight a secret society of evil. "Oh, I want to tell my feelings to Matsumoto-senpai, the man of my dreams, but only when I do so, I transform into an H battle suit."
During the late Tokugawa period, tensions between the bakufu and Chōshū clan are rising due to arguments over how to deal with foreigners in Japan. However, unlike the rest of the Chōshū clan members, Touma Kusaka wants Japan to develop good relations with foreign nations. To achieve this, he attempts to learn English behind his clan's back. A man named Keiichirou Akizuki decides to become Kusaka's teacher, but their relationship soon grows into something stronger, despite the fact that Akizuki works for Kusaka's clan's enemy, the Tokugawa bakufu.
