The Hatano couple had a problem. Akira was unable to get an erection during sex, and they had not had sex for over a year... To be honest, Yui was also suffering from a burning desire to have sex. The Kuroki couple lived next door to her in the same apartment building, and were a very free-spirited couple who were not afraid of being seen by others, and were very affectionate. One day, when Yui was passing by in the shared hallway, she witnessed the Kuroki couple having sex through the gap in the door...