From Viz:

The year is 1787, Tenmei Era, a time of plague and famine in Japan, When ordinary swordsman only add to the piles of corpses on the street. But Nakoruru, a nature-worshipping Ainu priestess; Haohmaru, a bold samurai with a death wish; Hanzo, a ninja loyal to the Shogun, are samurai spirits with a common destiny.

With Nakoruru's guidance, the trio battle the supernatural forces of the Dark Kingdom, led by the half-human wizard Madou Shiranui. If Shiranui's troops succeed in fomenting a war between Japan's warlords, the resulting devastation will clear the way for Dark Kingdom invasion of earth. Can the the brave samurai stave off an all-out attack by an entire legion of demons?