That Summer, I NTR'd My Friend's Girlfriend

The story begins near the end of summer vacation during the protagonists' college years. One day, the protagonist's roommate brings his girlfriend back to their apartment. He tells the protagonist that the two of them plan to move out in 20 days, right around the start of the new semester. Until then, she'll be staying at the apartment with them. The once peaceful shared living arrangement gradually begins to change from that moment on...​