During college, a drunk man wakes up in senior student Kokonoi's room and discovers her secret obsession: training her nipples through masturbation. The discovery draws them into a relationship no one else can know about. Years later they work at the same company and continue meeting in secret. Kokonoi's nipples have become even more sensitive than her clit, and she trains his nipples as well. He goes wild from a pleasure completely different from stimulation to his cock, while the sight of him makes her soaking wet. This is the climax of her obsessive record of extreme nipple training.