Shanaou Yoshitsune is an alternative story to the popular historical figure Minamoto no Yoshitsune along with his servants, including the renown Benkei who lived in the Heian Era. In this version, Yoshitsune was actually a weak, sickly boy who met an exact body replica in Hyouta, a traveling orphan who earn his living on the street. Being physically stronger, Hyouta was led to become a replacement for Yoshitsune, eventually claiming the name as his own.
This manga is a fictional story (albeit with many historical references) of his journey to restore the honor of Genji (Minamoto) Clan.
(Source: Easy Going Scans)