My father remarried and brought me a new stepmother, an older sister who is not much older than me. - With mixed feelings, the family gathers together under the kotatsu, skin touching each other in the kotatsu. - He gets horny and erects the body of a woman too young to call him his mother. - The other person feels the same way, so the two secretly flirt inside the kotatsu. - Eventually, they get serious about having sex.