Atsushi expects to spend a week alone while his live-in girlfriend takes a family trip. At her farewell drinking party, however, he grows dangerously close to her busty best friend Sachiko after his girlfriend passes out drunk. Once the trip begins, Sachiko returns and seduces him. They spend every day until his girlfriend comes home fucking and living together, with Atsushi repeatedly creampieing the one woman he was never supposed to touch.