Himari becomes his adult stepsister after their parents remarry. Her awful, cocky first impression makes him think they will never get along, but a certain event gradually brings them close. When she discovers he has a girlfriend, she mocks the introvert for having no experience and offers to be his practice partner if he promises not to penetrate. It starts with rubbing genitals, then one centimeter inside, then just the tip. Each excuse moves closer to real intercourse until her incredible body becomes a genuine practice toy and both drown in sexual pleasure.