Natsuka goes to her cousin Mitsuki's house every now and then. Natsuka is an only child and looks up to Mitsuki like an older sister. Today, she goes to Mitsuki's house again. Recently, her mother remarried, but her new husband made advances on Natsuka, which caused her to lose faith in men. While she was talking to Mitsuki about it, they ended up in a forbidden relationship... "Girls can have sex with each other, right?"