In the Dark Ages, after the fall of the Roman Empire, weak Britain is divided into several clans, while the powerful Ireland, untouched by the Romans, dominates and ravages the British tribes. The just and noble British leader, Marke, meets with the other clan chiefs to try to unite the country, but they are attacked and slaughtered by the Irish army headed by Morholt. Marke loses one hand protecting the young Tristan, who had just lost his parents, and Marke raises the boy as if he were of his blood.
Years later, after another attack by the Irish forces, Tristan rescues his people, who had been captured to serve as slaves, and kills the brutal Morholt, to whom the beautiful Irish princess, Isolde, had been promised in marriage. Tristan is poisoned by the sword of Morholt, declared dead by his mates, and put on a boat as part of his funeral. The ship floats out to sea, and lands on the Irish shore. Isolde, along with her maid-servant, Bradnae, finds the boat with Tristan in it, and hides him in a hut, where she nurses him back to health. In the process, they fall in love for each other, but with Isolde concealing her identity from Tristan. When Tristan is about to be discovered by Irish guards, he escapes by boat back to Britain to avoid being captured and killed. Isolde's father, King Donnchadh, together with the British traitor Wictred, declares a joust as part of a plan to divide the British and make Wictred king, by promising the hand of Isolde to the winner. However, Tristan wins the tournament. Without knowing that Isolde is his love, Tristan offers her to Marke, in order to make Marke king and thus unify the British clans. When Tristan sees who Isolde is, he must decide between his friendship and loyalty to Marke and his love for Isolde.
Henry Caville plays Melot, an old friend who betrays Tristan to his enemy.