One night, the protagonist found out his little sister was masturbating while thinking about him. Hearing her sweet moans broke him and he ended up raping her. That marked the beginning of their forbidden love, but his classmate, Satsuki, found out about it soon after. She wanted to stop him from making a big mistake, but Ami thought that Satsuki was trying to steal him from her. That's how the two girls started fighting over the main character.