From MangaHelpers:

During the feudal era, warlords were not only in danger from enemies outside the castle, but also from within like their own lieutenants they trusted. To protect themselves, they secretly began to use decoys of themselves. Honda Masanobu was part of Tokugawa Ieyasu’s council and also his closest friend. However, to defend his home country from Oda Nobunaga’s campaign, he leaves his position. During the 10 years on the battlefield, he meets Jirosaburo Motonobu who looks almost identical to his former master. Masanobu comes to realize that Jirosaburo must become Ieyasu’s shadow because of his trustworthy character and battle experience. [vyc]