親同士の再婚で突然出来た義理の妹。自分の意見を言えない内気な女の子、嫌なものは嫌だとハッキリ言えない気弱な女の子。そんな義理の妹に毎日のようにチクハラしていたら、感度が激上がりしてチクハラなしでは生きていけない体になってしまい、最後には自分から兄のボクの所にやって来て、あんなに自分の意見すら言えない引っ込み思案だった女の子が、自分の口で乳首を触って欲しいとお願いするようになるまでの成長記録。

My parents remarried, and I suddenly gained a stepsister. She's a shy girl who can't express her opinions, a timid girl who can't clearly say no to things she dislikes. But after I started harassing her with nipples every day, her sensitivity increased dramatically, and she became unable to live without it. In the end, she came to me, her older brother, on her own, and this is a record of how this once withdrawn girl, who couldn't even express her opinions, began to ask me to touch her nipples with her own mouth.