A group of Immortals quest to locate the mysterious "Source" in the near future. One member of the group, Zai Jie, breaks into a tower in Eastern Europe and contacts his associates to give them the location of the Source. The Guardian of the Source, who has supernatural speed, confronts and decapitates him. Reggie, another group member, discovers that the planets are moving from their orbits into a cosmic alignment.
Former Immortal-Watcher Joe Dawson is called by group member Methos to find their mutual friend Duncan MacLeod. Joe finds MacLeod in a fight with the Guardian, and shoots Duncan, hauls him into a truck, and drives off to meet with the others at a monastery, in order to meet with an ancient being known as the Elder so they can locate the Source.
At the monastery, they meet Anna Teshemka, Duncan's mortal wife, who is having visions. The Elder meets them all as a group, and tells of how, in ancient history, another group of Immortals sought the Source. Upon slaying the Guardian, the two survivors were cursed with decay, and one of them became the new Guardian. The Elder tells them all to follow Anna, who knows the way. She receives a vision from the Elder. Meanwhile, the Guardian comes and attacks Reggie and Joe Dawson on holy ground. In an effort to save Dawson, Duncan throws his katana at the Guardian, temporarily wounding him. The Guardian removes the sword from his neck and breaks it, killing Joe with the broken blade before escaping. After burying Joe, they leave for the Source, which they have determined to be on an island in a lake in Eastern Europe.
Approaching the island, the boat's captain tells them that the "maniacs" rule the island: cannibal gangs. After fighting locals who were poised to immolate a man to death, they obtain a van and drive to a deserted house a short distance away from the Source. That night, the Guardian kills Reggie by slashing him to death the Elder had warned them that the closer they get to the Source, the weaker they would become, and that they would lose their immortality. Duncan has replaced his broken katana with a pair of butterfly swords. The group takes Reggie's body with them, until they ultimately discover that he will not revive.
After burying Reggie, they continue on their way. They find the road blocked, and are captured by cannibals. While the cannibals are distracted by their drunken carousing, the Guardian frees Anna, and forces her to accompany him to the Source. Later, Giovanni also escapes and takes a sword, hoping to be "The One." Duncan frees himself and Methos, and sets off to rescue Anna. Giovanni gets recaptured, and Duncan goes in to save him. Methos saves Duncan at the last moment, and says he believes Duncan is "The One," due to his incorruptible nature. He rides off on a horse to distract the cannibals, and allows Duncan to chase after Anna. Giovanni, who had run off when Duncan came to his rescue, is decapitated by the Guardian.
Duncan finds Anna in a clearing by a sandy pit, illuminated by starlight, as the cosmic convergence is happening directly over them. The Guardian surfaces and challenges Duncan. Duncan finds he has the same speed and power as the Guardian, and fights him. Finally defeating the Guardian, he taunts Duncan to take his head. Duncan refuses, and the Guardian vanishes, screaming, in a blast of light, exclaiming that he is "cursed forever." Duncan enters the Source, qualified to do so by his pure heart. In the Source, Anna reveals that she is pregnant with their child. Duncan declares "He is the one."