Mio and Hibiki Shiina are step-siblings.

Hibiki began attending college the year after Mio left home. Seeing as Mio's apartment is close to campus, they began living together.

It's been about half a year since then.

One evening, Mio's boyfriend canceled last minute on her.
When she arrived home, she discovered her brother using her underwear to pleasure himself...

She doesn't recognize this person...

Read on as their relationship changes from "siblings" to something more!