all characters are at least 18
At the very beginning (Which can be skipped after seeing it the first time.), the three sisters come to visit the villa in the care of Inuzuka Nanao. Said villa is owned by the parents of the sisters, who are now living out of country. Since the girls and Nanao spend most of their childhood together, he later became the caretaker of the house. Anyway, Nanao enters the villa to discover they have arrived and encounters them one-by-one (This serves as an introduction to the personalities of girls and their relation to Nanao).
At the evening of the first day, first H between Nanao and the girls happens.
After that the game goes on until day 20, which is the time at which the girls have to leave again. Until then the Player is free to interact with them (As described in the gameplay section).
On the last day, if none of the girls had their affection maxed, they will just leave. Otherwise, the player can met up with one of them to experience an individual ending for each girl.