Ruka and Reiko are spending some quality time with their mom friends from the neighborhood. One of them casually mentions that "Ryuichi (their son)'s penis is about the size of an old telephone receiver," which intrigues the two. Furthermore, the mom friend has to go back to her parents' house and is worried about leaving Ryuichi at home. The two are desperate to see Ryuichi's big dick, and agree to take care of him for a week, knowing that they'll help each other out when they need it. A few days later, the day finally arrives. From that day on, a week of obscene sex like a string of beads begins...