He is seduced by his girlfriend's busty friend, who he should never touch, and has an affair with her! Tatsuya's girlfriend, who he lives with, is going on a family trip for about a week, and he will be alone for the first time in a long time. However, her best friend Sari comes to a drinking party she holds before she leaves, and while she is drunk and sleeping, he suddenly becomes close to her. After she goes on a trip, Sari comes to visit again, and for the week until she comes back, he and Sari start a creampie cohabitation life together, fucking every day...