The story explains the day-to-day of a college freshman Yūta, who is just starting a new life in the School of Literature Department of Tama University. While Yūta grew up with his sister, when she married a divorced man with two daughters, Yūta left her to live by himself to distance himself from that new family.

About 3 years later his sister asked him to visit and babysit her girls. His life was turned upside down when the girls' parents went missing. Yūta now has to live with his sister's three daughters: a tsundere (Sora, 14 years old), a little demon (Miu, 10 years old) and a baby girl (Hina, 3 years old), two of which are from a previous marriage and not related by blood, in a six tatami mat sized room.