In a post-apocalyptic future where global war has nearly wiped out humanity, the survivors live under the iron rule of the Oniken, a merciless cybernetic army. A small resistance led by General Zhukov fights to hold the machines back but is steadily being overwhelmed. Desperate, Zhukov turns to Zaku, a legendary ninja mercenary with a mysterious past who is said to be the only one capable of stopping the Oniken, and Zaku agrees to join the fight against the robotic legion.