Arisa has been married for a year and still loves her husband, having sex with him every day. But then her husband is suddenly sent on a one-month business trip. Thinking that Arisa will be lonely, her husband makes his sister live with Arisa during his business trip. However, her Sumire is actually bisexual, and has had her eye on Arisa since her Arisa started dating her, waiting for the right opportunity to draw her into the lesbian world.