Someone is robbing graves and sacrificing men in Duncan's homeland of Glenfinnan. In flashback to 1618, Duncan is in love with Debra Campbell, but she is pledged to his cousin Robert. Robert challenges Duncan and is killed. Years later, newly-Immortal Duncan fights Immortal Kanwulf to avenge his father. In modern day, Duncan brings back a bracelet he buried with Debra. He discovers the local priest is really Kanwulf, who threatens to keep killing locals until Duncan returns his axe.
Duncan and Joe arrive at the San Francisco airport, where Duncan encounters Immortal Andrew Cord being shot by a masked man - who turns out to be Charlie DeSalvo. Joe recognizes Charlie's intended Immortal victim as a former platoon-mate from Viet Nam, a soldier dedicated to maintaining unit solidarity and who saved Joe's life, but who is now selling defective weapons to Charlie & Mara's Balkan revolutionaries. Duncan is torn between loyalty to Charlie and Joe's loyalty to Cord.
Richie finds Immortal Mikey, a railroad-fascinated autistic man, wandering after Immortal Alan, Mikey's last protector, is killed. Richie and Duncan take Mikey in, but he is a trouble magnet, and the Immortal who killed his protector is now after him. Also, Duncan buys a decrepit house. In flashback to 1868 Dakota Territory, Duncan interferes with a white man beating an Indian boy.
Immortal Canus is using attack dogs to injure Immortals so he can easily take their heads, and Duncan is his next target. Meanwhile, Richie encounters the young man who killed Tessa and made him Immortal, and demands vengeance. In flashback to 1785 England, Canus has his dogs hunting a young boy and McLeod kills one.
In flashback to 1888 San Francisco, Amanda wins Immortal Kit's casino from him at poker. In modern day, still hating each other, Amanda comes to visit Duncan the same day Kit appears in town, desperate to buy a racehorse named Double Eagle. When Kit and Amanda wind up joint owners of Double Eagle, the fireworks start.
Kenny, the Immortal in a child's body, has returned. He runs to Anne for help when Immortal Terence Kincaid is after him, and she calls Duncan. In flashback to 1778 South Pacific, Kincaid is the self-absorbed captain of the ship Duncan works on. When mutineers want to behead Captain Kincaid, Duncan convinces them to maroon him on a deserted island instead. In modern day, Kenny and Amanda turn out to know each other. In flashback to 1182 England, Amanda finds newly-Immortal Kenny after his family has been killed by Normans and teaches him. In modern day, having Kenny ...
Duncan is kidnapped on orders of "the Colonel." Amanda takes a young thief under her wing who starts mimicking her appearance. In flashback to 1918 France, Duncan is unable to prevent glory-seeking immortal Colonel Killian from leading a charge after he's been told that the war is over, but Duncan's testimony convicts Killian at a court-martial. Duncan then asks for Killian's death sentence to be commuted to life in a criminal asylum. In present day, Killian's thugs kidnap Melissa thinking she's Amanda, and Killian leaves Duncan in a cell in an abandoned building.
Duncan and Richie foil an attempted murder by Immortal Kinman, but the intended victim's wife is killed by a stray bullet, and Richie doesn't understand why Duncan won't testify against Kinman. In flashback to 1712 England, Duncan is unable to prevent Kinman from goading a young friend into a duel and then killing him. Meanwhile, in present day, the victim's husband calls Duncan a coward for refusing to testify.
Duncan and Richie attend the opening of an exhibit featuring the Bengal Kali, a bronze statue sacred to the now-extinct Indian assassin cult of Thuggees. Immortal Kamir, last of the Thuggees, is determined to reclaim the Kali for India, and he wants Duncan's help. In flashback to 1764 British-Occupied India, Duncan is trying to track down the Thuggees and prevents young widow Vashti from committing suttee on her dead husband's pyre. In present day, Duncan discovers Kamir has killed a mortal to reclaim the Kali, but Kamir insists Duncan not judge him, for he is a ...
Immortal Walter Graham is trying to kill pre-Immortal Claudia Jardine, a brilliant pianist, so that she will remain young and at her creative peak forever. In flashback to England 1663, Duncan and Graham are traveling players, and Walter regrets his ability to recognize brilliance while lacking genius of his own. Meanwhile, Methos falls in love with Alexa and is devastated when Joe tells him she is terminally ill.
Duncan's Immortal friend Jim Coltec has killed one too many evil Immortals and has become the victim of a Dark Quickening. If Duncan has to take Coltec's head, will the Dark Quickening overpower him as well?
Duncan, still under the influence of the Dark Quickening, arrives in France. In flashback to 1917 France, Duncan is an ambulance driver and his Immortal friend Sean Burns psychoanalyzes the mental victims. In present day, Methos and Sean Burns try to get through to Duncan.
Duncan saves the life of President Hamad, a visiting politician, whose assistant is Immortal Kassim. But it turns out Kassim was arranging the assassination of the cruel Hamad to allow Nasir, descendant of the Moorish kings Kassim served, to take over the government. Now Kassim wants Duncan to kill Hamad in order to repay an old debt that Duncan owes him.
Magician Danny Cimoli is delighted to discover he cannot die and promptly incorporates being killed into his act. When Duncan tries to teach Danny about Immortals and defending himself, Danny doesn't take Duncan seriously.
In flashback to 1700's Scotland, Duncan and Immortal Warren Cochrane are among those fighting for Bonnie Prince Charlie. But only Duncan eventually realizes that the Scots must abandon the prince or be slaughtered in one mismanaged battle after another. In modern day, Goddard is paranoid and confused. He has forgotten he is Immortal. As Duncan helps him remember his past, he remembers that he blames Duncan for Charlie's failure to capture the Scottish and English thrones.
The de Valcourts are two Immortals who have been married, to each other, for two centuries. When their 300th anniversary turns into a marital battle and they separate, Duncan and Hugh try to get them together again.
Jack Shapiro tricks Joe into coming to Paris to face a Watcher trial. Duncan tracks Joe down, complicating Joe's situation even though he is trying to help. Meanwhile, Immortal Jacob Galati is tracking down and killing Watchers.
Continuation of "Judgment Day." The Watchers believe Duncan has been killing Watchers, with Joe's help. They are trying to kill both men, while Duncan tries to identify the real killer, another Immortal. In flashback to 1847 Europe, Duncan's Immortal gypsy friend Jacob Galati kills a mortal who raped Jacob's Immortal wife Irina, and he is promptly hanged by the gypsy-hating townspeople. In modern day, Jacob saves Duncan from a Watcher and tells him Horton's renegade Watchers killed Irina. Can Duncan and Methos stop the feuding before anyone else is killed?