Living together are husband Yoshio, his father Chiro, and wife Saki. Chiro constantly finds himself lost in lewd fantasies while secretly ogling his daughter-in-law's curvaceous hips. One day, while Saki is masturbating, a thorn suddenly pierces her buttock. She tries checking in the mirror but can't locate the exact spot. To make matters worse, her husband is away on a business trip with no return in sight. Desperate, Saki turns to her father-in-law for help—an innocent request that spirals into an unexpected and intense situation.