Yukina raised her son Masakazu alone until he graduated from university and secured a job. With Masakazu starting his own life, Yukina felt lonely living by herself. On Yukina's 50th birthday, Masakazu invited her on a hot spring trip as a present. Seeing Masakazu's body after so long, Yukina became excited. Masakazu also got an erection seeing his mother in lingerie. Where will their desires lead them?