A man is seduced by his girlfriend's busty friend, who he should never touch, and has an affair with her! His girlfriend is going on a family trip for a week, and he's going to be alone for the first time in a while. However, his girlfriend's best friend Hana-chan comes to a drinking party he hosted before she leaves, and while she's drunk and asleep, he suddenly becomes close to her. After she leaves on her trip, Hana, who has huge I-cup breasts, comes to visit him again, and they start a life of cohabitation, having sex every day for a week until she comes back.