Even after a year of marriage, Arisa is still deeply in love with her husband and they have sex every day. However, her husband suddenly gets assigned a one-month long business trip. Worried he'll be lonely, he asks his sister Sumire to live with Arisa during his absence. Unbeknownst to Arisa, Sumire is bisexual and has had her eye on her since Arisa started dating her brother, waiting for the perfect moment to lure her into the world of lesbianism.
