College student Yui Tomita went on a date with her boyfriend to a Tokyo amusement park. That night, they stayed over at her older sister's house, where her sister's husband—a handsome, youthful man in his late 40s—had always secretly been someone Yui admired. He was the complete opposite of her boyfriend, and seeing her sister's happiness stirred a quiet envy within her. However, she never imagined that she would soon be caught up in such an unexpected turn of events.