Nanase moved to Tokyo from her hometown of Okayama to attend university, but due to housing troubles, she ended up staying in an apartment where her older sister lived with her boyfriend. Nanase, who has been away from his girlfriend's parents and is curious about many things, ends up taking an interest in her sister's boyfriend, Yuta... Cute, big breasts, no bra! A dream-like story about being cuckolded by your girlfriend's younger sister who is getting more and more erotic!