Based on the visual novel by Waffle.

Sanjou Maki is the girl living next door to Sakonji Seiichi. She appears to be well-bred, gifted with both beauty and brains. However, one night, Seiichi sees her masturbating on the veranda...

Maki easily admits that she's a dirty girl and jokingly suggests to Seiichi that she should be blackmailed into being his sex-slave.