Your parents leave overseas for work, and you're home alone with your stepsister, Kurumi.
Kurumi is a bright and energetic girl who loves to tease you.
She knows exactly how to flirt and be naughty around her virgin older brother.
Even if it's just by marriage, you know you shouldn't get involved like that with your sister, but...
You might just give in to the cute Kurumi and cross a line that's forbidden to an older brother...