In the premiere episode, parapsychologist Dr. Sara Beckman is attempting to use a demon-summoning device, which serves its purpose all too well. The mysterious Raphael Cain shows up and rescues her, only to be accidentally shot by Sara for his pains. Goodwin whisks Raphael off, but needs Sara's car and apartment to buy him time to heal. The demons, led by Mallos and his ""assistant"" Vashista, see Beckman's device as a threat and try to track her down. Raphael manages to drive them off, but when he has to stop to rescue Sara, Goodwin is captured by the demonic duo.

In part 2, Mallos and Vashista torment Goodwin and use him as bait to draw in Raphael. The plan succeeds, but only after Mallos has sadistically ripped off Vashista's hand and sent her back to Hell himself. In their final battle, Mallos taunts Raphael with the fact that his daughter Kiyomi is still alive and out there...somewhere. He then manages to escape. Raphael and Sara recover Goodwin, and they decide to make Sara part of their team, bringing her scientific expertise on-board.

McQueen, an aging rock star, has a deprogramming center for demons on his farm. Raphael is skeptical about reforming evil, though Sara tries to see things from a scientific angle. Added to the mix is Randall, a deadly demon bounty hunter from hell whose agenda is to behead demons who have seen the light of goodness. In the end, it turns out to be a demon ruse designed to crush McQueen's hopes.

A demon body-jumper is the Immortal's newest threat: immune to his sword, it jumps bodies with impunity, including the body of Sara's romantic interest: a detective investigating the series of bizarre murders it leaves behind. Ultimately the trio track it to a hospital where it plans to take over the body of a senator and spread chaos from such a high position. Raphael discovers that the demon is vulnerable to electricity, and weakens it long enough to use his sword to send it back to Hell.

Deciding to take a break, the trio check into a ""fantasy suite"" type hotel, unaware that it is a haven for demon. Through the fantasy-decorated hotel rooms, guests are sent to a extra-dimensional region with the same motif, and used as a plaything for demons on ""vacation"". Sara and Goodwin take on the roles of a saloon girl and Billy the Kid, and Raphael must rescue them. He manages to break the enchantment over them, by wooing Sara and letting Goodwin-as-Billy shoot him down in a gunfight. He then goes after and defeats the head demon of the ""resort,"" freeing the rest of the prisoners from the spell and returning them to reality.

Goodwin is in love with a woman who is in cahoots with a demonic artist. They have occupied a loft that stands on a site of great magical power, and plan to use it to open the way to Hell. Filled with misgivings, Raphael and Sara have no choice but to let Goodwin find out for himself. It turns out that the demons need Goodwin as a sacrifice, but Rafe intervenes and sends them back to Hell through the portal they were ready to throw Goodwin into.

A mysterious death of a couch potato leads Raphael and Goodwin to cable studio D, while Sara investigates the victim's homelife. The demons have developed a way to broadcast Hellish transmissions that first suck unsuspecting viewers in to compulsively watching, then drive them to suicide. Tormented with visions of her dead parents (who were killed by demons), Sara almost falls prey to the transmissions as well. Meanwhile, Goodwin dodges the advances of an amorous soap star/female demon, and Raphael must fight his way past a series of TV-studio demon traps. As usual, in the end Rafe defeats the head demon, sending him back to Hell and defeating their scheme-of-the-week.

Blaylock, a demon who can change forms and favors plagues, breaks into a government research lab and steals a virulent new germ-warfare disease. He escapes ahead of Raphael, who is taken into custody by the FBI. He manages to convince the agent to free him and then help him track down the supposed terrorist. Meanwhile, Goodwin and Sara have taken the plane that the disguised demon is supposed to be on. It goes down in the woods near a reservoir, and they quickly find out that everyone on the plane are demons! Raphael arrives and manages to stop the demon just before he can dump the vial in the reservoir. The demons takes his form, but is no match for the Immortal's sword skills.

The theft of a jewel, the ""Bride's Kiss"", set the team on the trail of a demon who plans to use it to enthrall a millionaire/computer genius who has created a computer virus capable of destroying most computers. The trio manage to infiltrate the wedding reception and discover that the bridegroom, Maximillian, is a demon. Sara briefly falls victim to the mind-numbing powers of the ring, but Rafe and Goodwin manage to free her. Raphael sends Maximillian back to Hell and the world is saved.

Raphael is targeted by three demon assassins. He defeats the first one readily enough. The second, Oniko, is a female demon whom he briefly confronted early in his career. She tries to deceive him into trying to kill the third assassin, Randall (from the episode Half Way). However, it's a trap, and Raphael is attached to the generators at the local electrical plant. Meanwhile, Goodwin and Sara are looking for a permanent residence and run afoul of a demonic real estate agent who locks them up. They manage to escape and, overhearing the demons' plans for Raphael, get to the plant and free him. In the final battle Raphael sends Randall back to Hell.

The trail of demons takes Rafael, Goodwin and Sara to the northwest forests. Three nature spirits, sisters, share a crystal among themselves. However, one has turned to the side of Hell, become a corporate logging executive and is using foresting to try to draw out her other two sisters and gain their power. Meanwhile, the other two sisters seek to make Sara their third, while a wood nymph tries to seduce Goodwin. Ultimately Raphael recovers the crystals and defeats the evil sister.

Mallos returns with a deadly plan: having Raphael trapped in a mental institution, he does his demonic best to convince him he is not an immortal demon slayer, but ""Ralph"", an abusive drunken truck driver with impossible heroic delusions. The doctor in charge is unaware that he is serving the demons' cause. Goodwin and Sara (with the help of Lester - a homeless person telepathically tuned into the demon world) must save the Immortal from Mallos before his mind snaps.

When the team discovers a preppy boarding school is a source of demonic brainwashing, Goodwin and Sara go undercover to learn the truth and expose the demon agenda. Raphael finds himself bonding in the woods with a misunderstood teen on the run from the school. When Goodwin is co-opted into the demon student plans, Sara is nearly sacrificed before Raphael returns to save the day.

While on the trail of the demon Ashur, Raphael is put into a mystic coma, appearing dead to Sara and Goodwin. Goodwin is now faced with the responsibility of taking on the role of Immortal Demonhunter. However, this is all a ploy on Ashur's part. He transforms a rat into a duplicate of Lester Dash (last seen in ""The Asylum"") and tries to lure Sara and Goodwin to him. Meanwhile, Ashur's assistant Ellie turns against him and awakens Rafe from his trance. Goodwin is on the verge of defeat, but Raphael steps in and sends Ashur back to Hell. Still, Goodwin knows that if Raphael one day falls, he can take his place.

While on the trail of a demon, Raphael ends up inside a run-down, neighborhood bar. Unable to escape, he slowly discovers that he is locked in his own introspective voyage of emotional discovery until he is willing to admit to a few emotional realities. Goodwin and Sara are left on the outside to see if they came revive Raphael from this deadly trance.

While going through some old things of Sara's, she finds a photo of her grandmother from the 40's...posing with Raphael and Goodwin. In flashback, they tell the tale of how they met her. They were investigating a spa rumored to have mystical healing properties just after World War II. Not only is the spa a haven for various Russian and American spy factions, but it is also at the heart of a demon plan to use the waters of Hell to make those who attend the spa become demon-possessed. The two Immortals manage to save the day and pose with Sara's grandmother for a picture at the end.

The Immortal one, Sara and Goodwin are still in Europe when Sara is mysterioiusly kidnapped. The trail leads to a castle where a female demon has set up a ""fantasy resort"" for rich individuals. Demons take the role of fantasy characters like Hansel and Gretel, and then take over their rich guests and use them to spread evil. Goodwin and Raphael infiltrate the castle. While Goodwin is transformed into a frog, Raphael finds Sara trapped in the role of Snow White to the demon woman's wicked stepmother role. Raphael defeats the head demon and her minions, and the millionaires are freed.

The team travels halfway across the world to Eastern Europe when Raphael hears of a murder/kidnapping eerily similar to that of his wife and daughter's. Certain that Mallos is once again behind it, he's determined not to let history repeat itself and vows to reunite this father with his daughter. But they will have to overcome the hate and anger of a village torn apart by racial prejudice only to discover a few surprises along the way.

While travelling in Europe, Goodwin has dreams of a special girl. Raphael relates to Sara (in flashback) how they came to Europe once before on the trail of Mallos. He had kidnapped local Gypsy women, transformed pigs into men, and held the rest of the area in terror. A gypsy woman warns Raphael that he should not interfere, but he musters the Gypsy men and tries to lead them against Mallos. Meanwhile, Goodwin has become involved with a local gypsy girl, and chooses her and a normal life over Raphael. The Gypsy leader is captured, and ultimately Raphael surrenders himself to Mallos in return for the man's freedom.

If Raph, Goodwin and Sara can destroy Mallos as a human, they can stop him from ever existing as a demon. Ironically, by destroying Mallos, they set forth the very pattern that allowed him to make his ""deal with the devil"" and become a demon in the first place. With the help of an eccentric local (misunderstood) wizard, our heroes return to modern time to discover the devil's thousand year reign is about to begin. The key to stopping it rests on finding Raphael's missing child. In addition, this episode introduces KARAZAK, Mallos' demonic rival.

While strolling along on the beach, discussing their future, Rafe, Sara and Goodwin discover the teenaged Kiyomi lying on the sand. She has no memory of anything that happened to her since her disappearance 3 centuries ago. Meanwhile, Mallos and a resurrected Vashista are lurking nearby. Apparently Mallos' Millennia Plan require that he marry the Immortal's daughter. Vashista raised the girl in the Beyond, and still has motherly feelings toward her. She tries to come to an arrangement with Rafe: he kills Mallos and she gets Kiyomi. Kiyomi is eventually lured to the abandoned building where the wedding is to take place, and Rafe is captured. Sara and Goodwin sneak in, and Vashista ultimately turns over a wedding-gowned dummy to Mallos and frees Raphael. In a final battle, Rafe sends Mallos back to Hell and recovers his daughter. However, as the episode ends, Vashista shows up and warns that the arrangement is only temporary.