Miyako Kinjo lives alone in a house that resembles a hot spring inn. Her son, his wife, and her grandson come home for the holidays, but Miyako stubbornly refuses to take a bath with them. Miyako has a tattoo on her chest, which she wants to keep secret from her family. However, when her grandson Yuto finds out, rather than rejecting the tattoo, he becomes sexually aroused and ends up having a physical relationship with Miyako. Miyako is captivated by the young man's burning sexual desire overnight.