Yuriko accidentally discovers her son masturbating and is deeply stirred by the sight of his grown cock. Noticing he has started pleasuring himself in his bedroom, she quietly peeks inside—only to be caught in the act herself, as she is also masturbating. Both are shocked by the revelation. Soon, her son declares, "I can't hold back anymore," and Yuriko reluctantly agrees, saying, "Just this once." But one time isn't enough. Her son pleads, "This place is okay, right?" as he requests anal sex. At those words, Yuriko gives in and accepts him into her ass. Their desires intertwine, giving birth to a new, forbidden relationship.