StorylineEditDuncan returns to Seacouver. There, he finds the Immortal Cassandra waiting for him. She is being hunted by Roland Kantos, one of her students, and she tells Duncan of a "Prophecy;" that he will defeat great evil. Kantos relies on the hypnotic power of his voice to defeat his opponents, but Duncan, after nearly losing his head to Kantos's tricks, defeats the mesmerist by plugging his own ears.
Joe comes to the dojo and gets the cold shoulder from Duncan, but before he leaves, he tells MacLeod that Richie is in town, ("The End Of Innocence"). Duncan tries to make his peace with Richie, but the young Immortal is still plagued by nightmares after Duncan almost killed him in "Something Wicked" and warns Duncan. When Richie challenges and kills Carter Wellan, his mentor, Haresh Clay, comes for Richie, breaking his sword. Richie is arrested trying to steal another one from a museum, and is bailed out by Duncan, who warns him that Clay is as good as they come, and gives him a sword. The two Immortals work out their differences and, now acting as equals, it is Duncan who asks Richie to stand aside as he avenges his humiliation at Clay's hands when they first met, and takes his head.
In the episode Manhunt, Carl Robinson is now a baseball superstar, until the day an Immortal challenges him at the baseball field, and Carl is identified as his killer. A "Manhunt" begins for the Immortal, led by Matthew McCormick, Carl's teacher, who has a score to settle with Carl from long ago. Carl was a slave and his owner was McCormick's father-in-law. When Carl killed his owner, Matthew swore revenge, but MacLeod is able to persuade him to let it be and they arrange for Carl's very public "death", so that he can slip quietly away and begin a new life again. The series brought humor with the story Dramatic License in which a steamy new bestseller written by Carolyn Marsh, about the events from Duncan's past - events about which she should know nothing. Her book retells his story, using his name. Intrigued and alarmed, Duncan investigates, only to find that Terence Coventry, another Immortal, is also hunting Carolyn, but because she has made him the villain of the book. Less than flattered, Coventry wants revenge, but it is left for Amanda to discover the truth - Carolyn is Coventry's wife and is tired of his stories of the past, wanting to live with him in the here and now.
When Amanda disappears into a van after a robbery, Richie thinks she has been kidnapped and goes after her. He finds her with Cory Raines, an old acquaintance of Duncan's and Amanda's who has always brought them trouble. Unfortunately for everyone involved, the money that Cory stole belongs to a mobster, and he wants his money back. In flashback to 1926, Amanda leaves Duncan and their circus act to join Cory on a bank-robbing spree. (Money No Object)
An Immortal "Messenger" preaches peace between Immortals to Richie, who tries to convince Duncan to stop taking heads -- right when Duncan encounters the Immortal former commander of Andersonville prison, where Duncan suffered through part of the Civil War. Methos returns, and is amused to find the Messenger using his name, and is perfectly willing to let him if only to distract those who might be hunting for him. Joe discovers that the "Messenger" has left a trail of dead Immortals behind him, when those he convinced with his message of peace encounter those still fighting. Richie must decide if the price of peace is his head.
Duncan's old friend Ingrid turns up in Seacouver, she and Duncan one fought the Nazis together, but he's uncomfortable with her new mission. In modern day, Ingrid has assassinated a Russian politician and Duncan wants to stop her from killing her next target.
Duncan's friend Immortal Cassandra is after Marvin Koren, a vicious bandit from Duncan's past, but she tells Duncan he is also Kronos, one of the Four Horsemen, who brought terror and death to two continents for centuries during the bronze age. Cassandra reveals Methos was also one of the Horsemen which shocks Duncan to his core (Comes A Horseman). Kronos and Methos disappear, and neither has a Watcher. Uncertain of what to do next, Duncan and Cassandra enlist the help of Joe Dawson, at the beginning of "Revelation 6:8", to locate one of the two remaining Horsemen, Caspian, who is imprisoned in a Romanian psychiatric hospital. By the time they get there, they find Caspian gone, freed by Kronos. Dropped on his cell floor is a matchbook for a hotel in Bordeaux and the two Immortals go there, Duncan trusting it was a clue left by Methos. Methos arranges to met MacLeod without Cassandra and tells him the four Horsemen are now reunited to implement Kronos' plan to unleash a plague virus on the world if the world does not bow down before the Horsemen. He also tells MacLeod why Cassandra hates him so much - he enslaved her after she became Immortal.
Whilst MacLeod defuses the first viral bomb, Kronos kidnaps Cassandra and send Silas and Caspian after Duncan. MacLeod takes Caspian's head and escapes from Silas by jumping off a bridge mid-Quickening. Knowing that Kronos must be using laboratory animals, Duncan tracks a shipment of monkeys to an abandoned submarine base and confronts the three remaining Horsemen. As Kronos and MacLeod fight, Methos prevents Silas from killing Cassandra and takes the head of his former brother as Duncan ends Kronos' life. The twin Quickenings intersect dramatically as they rip through the submarine pens. Freed, Cassandra rushes to take the head of the weakened Methos, but is prevented by MacLeod. Later, Duncan accuses Methos of setting the whole thing up, to resolve his problems with Kronos and the two part, uncertain in their friendship.
In the season finale, they introduced a story arc involving Duncan being a chosen warrior against an ancient evil. Returning to the barge one evening from the Opera, Duncan and Richie are accosted by an elderly man who says he has come to warn MacLeod, but before they can speak to him, Duncan sees a familiar figure watching them - James Horton. Horton vanishes and the Immortals find the old man dead on the quay. The next day, they find Horton's grave opened, as Joe Dawson prepares to fly the coffin back to Chicago in secret. Despite seeing a body in the coffin, MacLeod is not convinced that Horton isn't dead. The man on the quay was Jason Landry, a noted archaeologist, and Duncan goes to see his grand daughter. They discover Duncan is named in Landry's papers as the champion who will fight and vanquish the demon Ahriman, who walks the Earth every thousand years. Unwilling to be cast in the role of the Champion who must face the "Archangel", MacLeod is taunted by Ahriman, who appears as Kronos and Allison Landry, who the demon kills. Richie seeks the help of Joe and Methos, who have their doubts about the existence of a Zoroastrian demon in 20th century Paris. As Joe and Methos talk with Duncan, Richie sees Joe as a prisoner of Horton and follows the illusion to an abandoned racecourse. He telephones Duncan to let him know, but rings off before MacLeod can tell him he is being deceived. Followed by Joe and Methos, Duncan goes to the racecourse and confronts Ahriman, who takes the form of Horton, Kronos and Richie, each in turn darting from the shadows at the Highlander. As they come, he strikes at them with his sword and takes the head of one of them, only to find that he has beheaded the real Richie. Joe and Methos arrive to find MacLeod kneeling over his dead student and, when Methos refuses to kill Duncan who offers his katana to him, the Highlander takes Richie's glove and walks away, leaving behind his friends and his sword, and vanishes.