"An oath sworn is an oath answered. An oath of vengeance for a life taken. A past destroyed, a future threatened. Enemy of darkness, he walks the earth, relentless. His mission, to hunt the messengers of evil, and drive them back to Hell. Now, the light of the earth depends on...the Immortal. With this opening narration, we find out about the backstory of Raphael Cain, a Dutch seamen stranded in Japan in the 17th century and who becomes married to Mikiko, a Japanese woman, and has a daughter. However, demons kill his wife and kidnap his daughter. His father-in-law, Yashiro, gives him the power to swear an oath that so long as there are demons walking the Earth, Raphael shall live as long as it takes to send them all back to Hell. Accompanied by his equally immortal "squire," Goodwin, the two have been around 300 years. Now, allied with a parapsychologist, Dr. Sara Beckman, they battle demons in the modern-day and hope that they will be able to recover Rafe's daughter Kiyomi.