Alone and with nowhere else to go, Nobuhiro moves into the house of Mr. Kisaragi, who also lives alone. Then, Mr. Kisaragi said, "From today, I will be your father, mother, and sister!" and their new life began. However, the older sister is very sloppy around the house, wandering around in her underwear, and hugging Nobuhiro easily...which makes Nobuhiro's adolescent heart pound every day.