Saki, a mother who secretly yearns for her son Kosuke, indulges in masturbation. Meanwhile, Kosuke is also harboring feelings forbidden to him by his mother, and is tormented by them. As they spend their days unable to confess anything to each other, one letter changes their relationship. Saki holds Kosuke in her arms as he becomes upset after learning that they were switched at birth. Taking care of the emotions that they had been suppressing, the two of them bond stronger together through insemination sex, replacing the bond between mother and son...