Mako Oda, who became the new stepmother through her marriage to his father, is deeply hated by her stepson Yuki. One day, Yuki forcefully rapes Mako and records the act on his smartphone. Using the video as leverage, he begins training her as a titty-fuck sex slave. His training grows increasingly extreme, and on nights when her husband is away, Yuki teams up with a delinquent senior to restrain Mako. They bring her to climax repeatedly using electric massagers and vibrators, then coordinate with the well-endowed senior to assault Mako's large breasts. Gradually, Mako can no longer endure the torment and begins orgasming uncontrollably.