Namino Yuuta enrolls into a school under his mother recommendation.
This school used to be an prestigious all-girls school, but because of the declining number of students, it had no choice but to go coed.

Yuuta thought there would be a few male students here and there but to his surprise, he is the only male in the whole school.
Not only that but the boy's dorm is all the way up a mountain behind the school. Yuuta a very shy kid feels uneasy and depressed about the future.

At least Yuuta's aunt, Risako whom he's loved since he was a little kid, works at the boy's dorm as the dorm mother and caretaker.
One day, Yuuta arrives at the dorm earlier than usual, and finds Risako bound in a S&M rope play.

"Yuu-chan, stop, don't rape me"- Risako says, fantasizing.
After hearing that, Yuuta can't control his feelings anymore.