On a certain night, the protagonist witnesses a school girl being forcibly dragged away in the shopping district of the city. That girl happened to be one of his students, Kandachi Kanami, who would always look down on him. Without thinking, the protagonist saves Kanami from the thug, and takes her back his home.
When he asks Kanami the reason she was being harassed, she suddenly makes a request of our protagonist to let her stay one night at his place. It becomes clear to the protagonist, seeing Kanami with a large bag with her late at night in the shopping district, that she has run away from home. Seeing the usually haughty and rude Kanami helpless like this wakes his inner demon, and fueled by his desire for revenge, he demands Kanami to sleep with him. She resists, but he throws her onto his bed, and forces himself upon her.
Come next morning, even after what happened, Kanami still does not show any signs of leaving the protagonist's home, as she has no one else to turn to for help. The protagonist originally planned on apologizing for he did the previous night, but the situation ended up exciting him all over again.
This marks the start of a life of the two together where the girl pays back the man in "installments", using her body.