Signal, a robot companion built for Nobuhiko, is attacked by Pulse, one the its creator's previous prototypes. Reprogrammed by Dr. Otoi's mysterious rival, Pulse is loaded down with weapons and sent on a mission by Otoi: steal the most recent robot technology, and obliterate anything that gets in his way.

The year is 1580 and Nobunaga Oda is working to consolidate his power over Japan. In his army are weapons of great destructive power: rapid fire machine guns, lasers, missiles and, most frightening of all, the reputedly immortal ninja Ginnai Doma. Like a sentence of divine wrath he hunts down and destroys dissident monks, ninjas and others, wherever they may be. With the clans on the brink of destruction, a single ninja accepts the impossible task: kill the immortal Ginnai.

When he was young, Arimasa Tsuchimikado rescued an injured cat and wished for it to become human. The cat, Kagetsu, is actually a member of the shape-shifting Kouga tribe and comes to live with Arimasa.