Koshimizu Sou lives with his widowed mother Sanae. Sou is incredibly short, far too short for a boy of his age so his mother agrees to help him do exercises and stretches to help his body grow. Doing stretches with his mother makes him incredibly excited and he can't hide his erection from her. Sanae takes it on herself to relieve her son whenever he gets too excited as they exercise together. Eventually the excitement gets too unbearable for both of them and they cross a forbidden line together.