"If you call me big brother, I'll let you c**." Fumi, a college student, is 20 years old and has never had a girlfriend, so he decides to try out a dating app, and the person who shows up to meet him is Tomo, a handsome hunk. "Fumi, does it feel good?" His long fingers tease his nipples and bu**... He was satisfied physically and mentally, but it was just a one-night stand. One day, Tomo shows up as the son of Fumi's mother's boyfriend...!? A secret cohabitation love story begins with a playful, sweet, sadistic stepbrother and a naive, cute stepbrother!