After being told by their parents about a remarriage and having to live together, the new stepsister immediately captures attention with her adorable charm and voluptuous body, feeling like a scene straight out of an anime. However, she openly shows disgust, saying, "You're my brother? Gross... We're not actually related by blood, right?" One day, sneaking into her room, the brother catches her masturbating, and from that moment, their relationship takes a sudden turn. Soon after, they become intimately involved as secret lovers. As long as there's no blood connection, having sex doesn't feel so wrong—somewhere deep inside, that thought begins to take root.