When Junpei is to go on a one-night onsen trip with his aunt Mizuki instead of his mother who sprained her ankle, he can't help but fantasise about having having sex with the kind, sexy aunt. Still, he does not really expect anything to happen, but when they enter a private bath together, he gets his hopes up. However, never had he imagined that Mizuki, lonely due to her husband having an affair, would ask him to be her lover, let alone the relationship that will ensue.