Ken is staying overnight at his childhood friend Hajime's house while his parents are away on a two-week trip. Then, suddenly, Hajime's older sister, Rika, who is also a married woman, comes home angry after fighting with her husband. Ken, who has been in love with Rika, a beautiful woman with big breasts, since childhood, witnesses the scene where Hajime and Rika are brother and sister and are affectionate with each other. Shocked and stunned, Ken stands there, and Rika gives him an erotic look...!