Tsubasa lives with her husband's father, her father-in-law, and their marriage has been going well. One night, she and her husband try to spend intimate time together, but as always, he finishes too quickly—before she can reach satisfaction—and immediately falls asleep. Left unfulfilled, she has no choice but to pleasure herself. Late at night, Tsubasa feels a throbbing sensation in her lower body and quietly begins moving, trying not to alert her father-in-law. But he notices and starts touching her intimately. Shocked, she tries to cry out, only to have her mouth covered. "My son can't satisfy you, can he?" he whispers, bringing her to climax with his fingers. Then, he pounds into her with fierce thrusts, making her climax repeatedly. The experience with her father-in-law is shockingly intense compared to her husband, and Tsubasa gradually becomes obsessed with him. Eventually, she starts willingly going to his bedroom on her own. Her affair with her father-in-law becomes a forbidden gateway, pushing Tsubasa past a new boundary in desire.