Yukina single-handedly raised her son Masaaki until he graduated from university, and successfully secured a job. - Masaaki started living alone when he got a job, and Yukina ended up living alone and lonely. - On the day of Yukina's 50th birthday, Masaaki invites her to go on a trip to a hot spring as a birthday present. - Yukina gets excited when she sees Masaaki's body for the first time on a trip. - Masaaki also got an erection when he saw his mother in her underwear. - Where will these two end up...