Jun'ichi Nagase attends a prestigious high school. He has the nickname Geno Killer since he was rebellious in middle school. This is used, inadvertently, to help a girl named Yuuhi Katagiri from trouble. She later transfers to his school, and in order to protect Yuuhi from danger, Jun'ichi is forced to kiss her. Not understanding what happened, she screams at him. Subsequently, it turns out that Yuuhi is his fiancé as arranged by their parents. Their parents discuss the matter and order them to go out together for a month to restore their relationship. If their relationship does not get better, the engagement will then be canceled.

(Source: Wikipedia)