Hyakkimaru is just a man who lacks 4-8 areas of the body since they were shot out of him by demons before arrival, as payment from his father, Kagemitsu Daigo. When the baby boy was born he had been missing 4-8 components of the body, and thus was abandoned--pitched in to a river. Hyakkimaru has got human body parts so he could eradicate the 4-8 demons which were produced out of his body, and also to regain his lost parts and has grown up. And the adventure is Dororo, the boy thief, with whom he becomes friends.