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. The cheeky older sister, Ayane, and the laid-back younger sister, Kotone. With the two always around, Kazushi has been semi-forced into a life of abstinence for a long time... One day, a trivial argument leads Kazushi to take a bath with the sisters in the apartment's tiny bathroom. Even though he is still wearing his swimsuit, the soft touch of them clinging to his body makes Kazushi's reason waver.

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. Inspired by the glimpse of her bare legs peeking out from under the covers, Kazushi starts to touch Kotone's body again as they sleep together, thinking that she forgave him once. However, the person he thought was Kotone and put his hands on was actually Ayane.

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 now has the most awkward relationship, appears before him. When Kazushi apologizes cordially in front of his apartment, Ayane surprisingly doesn't seem angry. On the contrary, the moment she enters his room, she asks him with tearful eyes, "If you want to continue what happened yesterday... give me a kiss..."

"Because you're like family to me... I can always be by my big brother's side," Kotone mutters sadly as she steals Kazushi's lips. As the sports festival draws to a close, Kazushi learns the girl's true feelings in an empty classroom bathed in the sunset. "I don't mind being second... just look at me for now," Kotone says as she tries her best to convey the words, grabbing Kazushi's hand and making him touch her chest. With his heart beating painfully fast, Kazushi comes to a conclusion...

Kazushi and the Shirakawa sisters decide to go to the pool on the substitute day off for the sports festival. Ayane, who has been tasked with preparing the lunch boxes, finds herself standing in Kazushi's kitchen wearing only a nude apron for some reason. "I heard from a friend that this is the best way to seduce a guy..." As she blushes and shakes her buttocks while they're alone in their apartment, Kazushi can't help but feel a sense of relief. He slowly approaches her from behind...

"My swimsuit is... gone!?" Kotone's bikini falls off the moment she slides down the water slide at the pool. Kazushi quickly hides her chest and runs into the storehouse, but the sight of the two of them alone topless is too stimulating for a boy of his age...