The film is set in Britain and Ireland in the Dark Ages, after the fall of the Roman Empire. Lord Marke of Cornwall plans to unify the people of Britain – Celts, Picts, Angles, Saxons, and Jutes – under himself as high king to resist Irish domination. The Irish king, Donachadh, discovers this and sends troops to attack Tantallon Castle where a treaty between the British tribes is being discussed. The raid claims the lives of the castle’s lord and his wife, but Marke manages to save their young son, Tristan, at the cost of losing a hand. Feeling compassion for the boy, whose father loyally supported him, Marke welcomes Tristan into his home and regards him as a son.
Nine years later, Tristan has grown into a fierce, courageous warrior whose loyalty to Marke is that of a son to his father. Tristan and other Cornish warriors launch an attack on an Irish slave caravan. Tristan fights Morholt, the leader of the army, his father’s killer and the man Donachadh’s daughter, Princess Isolde, has been promised in marriage to. Though Tristan kills Morholt and Donachadh’s forces are overrun, he is severely wounded in the fight and believed dead, though he is in fact only suffering the effects of Morholt’s poisoned sword. Tristan’s body is put out to sea on a funeral boat which eventually washes up along the shores of Ireland. He is discovered by Isolde and her maid, Bragnae, who administer an antidote that revives him. Bragnae insists that Isolde conceal her identity so Isolde tells Tristan her name is Bragnae and that she is a lady-in-waiting. Tristan and Isolde fall in love as she nurses him back to health.
The two lovers must separate after Tristan’s boat is discovered. Tristan returns to Cornwall and receives a hero’s welcome. An overjoyed Marke welcomes him back with open arms. Plotting to defeat Britain, Donachadh proposes a peace treaty, promising Isolde in marriage to the winner of a tournament. Tristan wins the tournament on behalf of Marke, unaware that “the prize” is the woman he fell in love with in Ireland. When he discovers the truth about Isolde, he is heartbroken to see her betrothed to Marke, but accepts it since the marriage will end the bloodshed between their two peoples.
Marke is kind to Isolde and genuinely falls in love with her. Isolde grows fond of him, but her heart still belongs to Tristan. Tristan is cold and distant towards Marke, who is confused over what is tormenting Tristan. Isolde tells Tristan that she is his anytime he wants. Tristan is torn between his love for Isolde and his loyalty to Marke. Tristan eventually gives in to Isolde; they renew their love and begin an affair. The affair is discovered by Lord Wictred, a Saxon chieftain and a longstanding dissenter to Marke’s leadership. He informs Donachadh and they conspire to use the affair to overthrow Marke, with Wictred getting Marke’s throne in exchange.
Marke confides in Tristan that he believes Isolde is having an affair. Tristan is tormented by the guilt and burns down the bridge where he would meet Isolde. After Marke and Isolde’s coronation, Tristan attempts to end their relationship, but Isolde begs him not to leave her. They are caught in an embrace by Marke, Donnchadh, and the other British kings. Donachadh pretends to be furious that his daughter is being used as a “whore” and ends the alliance. Seeing this as weakness on Marke’s part, the other kings also decide to part ways with him. Marke is hurt and furious over Tristan and Isolde’s betrayal. However, after Isolde explains their history, Marke relents. Tristan is taken to the river and Isolde tells him that Marke is letting the two of them and Bragnae leave together. Tristan puts Isolde in the boat meant for their escape and tells her that if they leave they will be remembered for all time as those “whose love brought down a kingdom.” Tristan pushes the boat away from the shore and runs off to the ensuing battle.
At the same time, Marke’s nephew and Tristan’s close friend, Melot, resentful of his uncle’s long favoring of Tristan, shows Wictred an old passage into the Roman foundations of Marke’s castle. Wictred had made Melot believe that he will become king when Marke is defeated. Once they are in the passage, Wictred stabs Melot and sneaks his army into the castle. Marke and his forces swiftly become pinned down by Donachadh’s army outside the castle and Wictred’s men within.
Tristan sneaks back into the castle via the secret tunnel, which he and Isolde used to carry out their affair. On the way, he finds a dying Melot; the old friends forgive one another before Melot dies. Tristan attacks Wictred’s men, allowing Marke’s soldiers to secure the castle. Tristan is mortally wounded in combat by Wictred, whom he still manages to kill. Now outnumbered, Tristan, Marke and the soldiers loyal to him emerge from the castle and present Wictred’s severed head to Donachadh. Marke urges the British kings standing with the Irish to aid them in making Britain a single, free nation. Inspired by his words, the British kings and their men attack Donachadh and his army.
As a fierce battle between the British and Irish erupts, Marke carries a dying Tristan to the river, where they are met by Isolde. Marke has to leave to lead the British to victory, leaving Tristan and Isolde to say their final goodbye. Before he takes his last breath, he tells her: “You were right. I don’t know if life is greater than death. But love was more than either.” Isolde sees to Tristan’s burial beneath the ashes of the Roman villa where they had met to be with each other. She plants two willows by the grave, which grow intertwined. She then disappears from history and is never seen again. Marke went on to defeat the Irish, unite Britain, and rule in peace until the end of his days.