A new erogenous zone, the "Pirum Muscle," has been discovered in the female buttocks! [What is the Pirum Muscle? It is a muscle located in the deepest part of the buttocks, closely connected to the nerves. With special stimulation, it becomes the body's ultimate erogenous zone for women, surpassing even the nipples and clitoris.] Nao Wakana, with her exquisite body, takes on the challenge of Pirum muscle development for the first time in AV history. Though skeptical at first, as she receives a special massage from an expert, her crotch gradually becomes wet! When the pleasure exceeds her limits, she shakes with spasms, reaches a climax that makes her wet herself, and cums repeatedly through Pirum sex! The mystery of the female butt is revealed for the first time!