Aki and Kazuki, two high school friends, discuss their upcoming beach trip and head to Aki's house to plan. There they meet Aki's older sister, Misaki. To Kazuki's surprise, Misaki was fresh out of a shower, wearing only a towel around her very curvaceous body. 

Aki clearly doesn't care and simply wants to continue planning the beach trip but Kazuki is extremely flustered. When Misaki asks him to help her with cleaning her room, since she has just arrived back from college, she begins to reminiscence things about high school. 

Through her reminiscencing it is learned that Misaki asked Kazuki, a fairly popular student, to befriend Aki as he consistently got bullied by girls. Misaki took it to the extreme and said that she would let Kazuki play with her tits if he did. Thus, Kazuki befriended Aki and got to play with Misaki's tits everyday. 

This reminiscencing led to Misaki suggesting to go back to the old times, which Kazuki gladly accepted. Which led to them having sex, because why not. Kazuki learns that he's the only man that has ever touched her and once Misaki proposes that they can continue during summer since she doesn't have school. 

Kazuki and Misaki's fun time have been recorded many times by Aki. Aki jacks off to these tapes and forces Misaki to continue doing it with Kazuki. The thought is that maybe it's because he's jacking off to his sister but Misaki thinks it may be for the other party involved in those tapes...