The eldest son, Hideki, is reliable, and the second son, Kenji, is a spoiled child. They were both born from Eiko's easy-to-give-birth-to hips, but they have different personalities. While Kenji is not afraid of being seen and is pampered by Eiko's hips, Hideki, as the eldest son, suppresses his own desire to be pampered... One day, Kenji accidentally peeks at Eiko's dildo masturbation, which is a daily routine to maintain her easy-to-give-birth-to hips. Seeing his mother's big butt, Kenji's love for butts explodes and he approaches Eiko's butt...