In a hillside castle surrounded by the chaos of war, strange disappearances are occurring and the noble Princess Yuki fears for her life. One lone swordsman, Samanosuke heeds the princess's call for help and rushes to her side, only to find she has mysteriously vanished. Outside the fortress walls, enemy troops attack the attack the castle and now Samanosuke must stand by his vow to avenge the princess and defeat a legion of demon warriors.

Genma Onimusha is an enhanced version of Onimusha: Warlords. It retains the content of the original release and adds a few new areas with a new boss enemy. It also features new costumes and body armor. Some of the enemies are placed in different locations compared to the earlier version.

This release also introduces three-tier charge attacks for each weapon and green souls gameplay mechanic. Accumulating a sufficient amount of green souls allows the player character to slowly regenerate health. However, if an enemy takes possession of the souls he will gain new attacks and his defense parameter will rise.