Jun'ichi Nagase earned the title of Geno Killer for his rebellious nature in middle school, but as a student at a distinguished high school, he becomes known for something else entirely. When a girl named Yuuhi Katagiri requires his assistance, his past notoriety inadvertently comes in handy. Upon Yuuhi's transfer to his school, Jun'ichi feels duty-bound to protect her from harm, resulting in an unexpected kiss. However, Yuuhi reacts with confusion and even anger towards Jun'ichi. Their true relationship is revealed when they learn that they are actually betrothed by their respective parents. In an effort to improve their connection, the couple is compelled to date for one month, with the threat of potential cancellation of the engagement looming over them.
