Based on the erotic game by Noesis.

A kind and reliable older sister, Mizushiro Ayaka.
A younger childhood friend and now my girlfriend, Tsurutani Ayaka.
While I'm happily getting along with the two, I wasn't satisfied. That is because I have a hobby that I cannot let surface.

I want to do, S&M.

I sneakily buy tools for my personal use, stare at them, touch them. One day as I was expelling myself of my urges, I noticed that someone had entered my room, and closed on to that secret.

"Let's try setting a trap"

I expose who discovered that secret. And so, our abnormal relationship began.

(Source: Noesis Homepage)