"I fell in love with my son as a man." Yusuke has come to Tokyo to attend university, and once a month Yui, his mother, visits him at his apartment to look after him. However, as she visits him again and again, her love for her son gradually changes into feelings for a man of the opposite sex, and they make a promise that should never be crossed. "There's no way this relationship can be tolerated. If we don't see each other for a while, we should be able to go back to the way we were as parent and child, and that's definitely a good thing... but is that really okay?"Yui continues to ask herself this question, but the throbbing in her body is more intense than she could have imagined, and this month she will do it again...