Ritsuka Aoyagi is a sixth grader who's troubled by the loss of his older brother. One day he meets a mysterious man who claims to have fought alongside his brother. His name is Sobi Agatsuma, who wants Ritsuka to become his new sacrifice since his brother died. When Rituka accepts, the mysteries of his brother's death become clear. Ritsuka then steps into a violent world of struggle against the enigmatic organization known as the Septimal Moon.