Her husband, remarried, always comes home late, leaving Yumi starved for affection. Filled with loneliness, she finds solace in masturbation, eventually turning her desires toward her stepson, Masanori-kun. One day, she discovers him passionately masturbating with goggles on. Unnoticed, Yumi begins pleasuring herself, but can't resist any longer—she mounts him, riding his bare cock in a cowgirl position. Overwhelmed by mounting loneliness, the stepmother overpowers her stepson, and the two impulsively dive into a forbidden relationship. Sharing the secret from her husband, a sense of complicity grows between them, drawing them deeper into the thrill of their hidden trysts. Before long, they're willing to risk pregnancy, surrendering to creampie after creampie.