我是姐姐的玩具

Aonuma Aira is a 13-year-old with no romantic or friendly relationships, but one day, due to his parent's remarriage, the high-schooler Misa becomes his step-sister! She's a completely unknown entity to him, and gets way too close way too fast. Moreover, her hobby is... makeovers? So she starts making him over until he's turned into a pretty girl! What's in store for him after he's become his step-sister's plaything?