Our hero Masaki and his sister Mami are overjoyed when their parents reconcile their differences, because that means they can be together again. However, when Masaki sees his sister's now voluptuous breasts, his body gets aroused.

One night, he notices a crying kind of noise coming from his sister's room. Worried that she might be crying over something Masaki peeks inside, only to find her playing with herself! Masaki finds himself unable to keep his hands off himself at the sight.
Realizing he can't possibly tell her about what happened, Masaki pretends everything is as usual the following day. But that night isn't like the night before...

[Edited from English DLsite]