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