Miyako Kinjou lived alone in a quiet house resembling a tranquil hot spring inn. When her son, daughter-in-law, and grandson visited during a long holiday, Miyako declined joining the family bath. The reason was a tattoo etched on her body—a secret she had kept hidden from her family. However, her grandson Haruto discovered it. Rather than rejecting her, Haruto became sexually aroused by the tattoo and began a physical relationship with Miyako. Overwhelmed by the young man's passion, Miyako found herself completely captivated by him in just one night.