This is the fourth installment of the new "Let's Have Sex Together" series, offering a realistic, first-person perspective on having sex with girls! You are hard at work studying when your live-in stepsister, Yui, returns home with her friend Aoi—both of them completely wasted. Unable to concentrate on your studies amidst their drunken rowdiness, you go to complain, only to get yelled at and end up giving them a massage instead. As Yui begins to get aroused, Aoi gets carried away and starts stripping off her clothes...