The Reason Why I was Cheated on By My Girlfriend

That day, I, Haruki Ogura, left my notebook at school. Since I have a homework due tomorrow in that notebook, I decided to go back to school at night to get my notebook. However, when I was about to enter my class, I saw a scene that haunts me to this day. “Ahnn…hmphhh…this is the best♡” It was my childhood friend, Niimi Yua, embracing a stranger, half naked… Is that really Yua? What is happening?