"Don't get the wrong idea just because we did it once!"

Asato, a male college student who is constantly working part-time jobs, hears the voices of a man and a woman arguing in an alleyway after work one day. When he looks in... he sees Yukino, a classmate he admires who attends the same university! Asato helps Yukino without thinking. However... for some reason, Yukino takes him to a hotel, and he crosses the line at her invitation...!?
Since that day, Asato has been worried about his relationship with Yukino, but his father remarried and Yukino became his step-sister!?
(MTL)