Yosuke is Haruka's boyfriend and they are living together. Yosuke and Haruka work different hours so they have very little time together. Yosuke asks Haruka for permission to have sex today because it has been a long time since they did it. Haruka agrees to let Yosuke indulge when he comes home from work. When he comes home, he finds his girlfriend sleeping, wearing very sexy underwear that Yosuke can't resist. While they are having sex, he suddenly sees the face of the girl under him, it's Kanon-chan, his girlfriend's little sister! It turns out she had a fight with her mother today, so she came here to hide. Luckily, she won't stay long, by tomorrow morning when Yosuke comes home from work, she will have gone home. Haruka promises to make it up to him for today. When Yosuke gets home, he sees a naked woman wrapped in just a towel coming out of the bathroom. Thinking it's his girlfriend because Kanon should be gone by now, Yosuke doesn't hesitate. He quickly dives into her, releasing all the pent-up frustrations. But in the heat of the moment, the towel falls off, and the girl he's having sex with is actually Kanon! Shocked and confused, he loses control and ejaculates inside his girlfriend's little sister...