Kazushi, a university student living alone, is troubled by the presence of two childhood friends, sisters, who hang out all the time at his apartment.
That night after their secret affair in the bathroom, Kazushi is unable to sleep due to excitement and feelings of guilt. While Kazushi is like that, Kotone and Ayane are sleeping soundly in a row as usual.
The day after a depraved night with his sisters, Kazushi is walking along, reflecting on the events of the previous night, when Ayane, with whom he
"Huh... you were lonely without me...?"
"Because you're like family to me... I can always be by my big brother's side,"
Kazushi and the Shirakawa sisters decide to go to the pool on the substitute day off for the sports festival.
"My swimsuit is... gone!?"