According to Dr. Jason Landry's research, every thousand years, the Zoroastrian demon, Ahriman returned to wreak havoc on the Earth. He had last been defeated a millennium ago by an Immortal named Timothy of Gilliam.
Archaeologist, Jason Landry warned Duncan MacLeod that he was the champion chosen to do battle with the forces of evil at the end of the 20th century. Duncan was skeptical, until he saw people he had killed, such as James Horton and Kronos, alive again, as aspects of the demon.
Ahriman took the form of Jason Landry after killing him. Jason's granddaughter, Allison Landry, tried to help MacLeod, and she then was killed by Ahriman in a fire, making it look like an accident. Ahriman would take the form of Jason and Allison Landry to try to taunt MacLeod.
Joe Dawson and Methos feared Duncan was losing his mind, but he was championed by Richie, who speculated that the eternal conflict against this demon may well be the reason immortals existed in the first place. MacLeod's struggle with Ahriman reached a peak when the demon posed as Richie at the same time as the real Richie was approaching, sword drawn, in order to help him. MacLeod accidentally killed Richie Ryan, thinking he was Ahriman. Sobbing, he then tried to have Methos kill him, but the ancient flatly refused.
Duncan fled Paris, and retreated to a monastery. He remained missing from the Watchers' database, and even Dawson couldn't find him. Duncan returned to Paris after a year of mourning and meditation, to visit Richie's grave and reconnect with Dawson. He was again confronted by Ahriman, who was disguised as Kronos and Horton respectively.
Duncan vowed to defeat Ahriman. A mysterious woman, Sophie Baines, who was an expert on the demon and knew how to defeat it, turned up. But as it happened, she had been previously lured to her death by the demon, and her temporary resurrection was its doing, but she guided Duncan all the same.
Ahriman tried to use Sophie's brother, Andrew Baines, to lure MacLeod into a trap by threatening Sophie. For assistance, MacLeod turned to Father Beaufort, and Ahriman then took the form of Father Beaufort's brother, Jackie Beaufort, who had committed suicide years ago, to try to get Father Beaufort to lose his faith and kill himself, but with the help of MacLeod, he was saved.
MacLeod discovered the right path and defeated Ahriman in the end.