Futabo

Wakana Seito lives in blissful confusion. His father remarried to a beautiful young woman named Kada, but passed away not long after. Now Kada has raised Seito on her own, and fifteen years later, he has grown to see his step-mother not as a parent, but as a beautiful woman instead.

Their relationship as mother-and-son maintains itself until one day Kada's twin sister Kanna comes to visit. Seito awakens from an afternoon nap and spies his mother in the shower, but his step-mother isn't upset at all to catch him spying on her—and she's very glad to see him.