As war rages between Cornwall and Ireland, the Cornish knight Tristan defeats Morholt, a formidable Irish warrior, in combat. Tristan himself is gravely injured and drifts out to sea, eventually landing on the coast of Ireland. He pretends to be a minstrel named Tantris and is treated at a convent by the Irish princess Isolde, who is disguised as a maid. As Tristan recovers, they fall in love—even after Isolde examines his sword and deduces that he killed Morholt—and when he is called back to the Cornish court, he promises to return for her.
Mark, the king of Cornwall and Tristan’s uncle, plans to establish peace through a political marriage to Isolde. Tristan volunteers to escort the princess to Cornwall. When he arrives in Ireland again, he and Isolde are each astounded to learn of the other’s true identity. Tristan, who is fiercely loyal to the king, rejects Isolde’s suggestion that they run away together. She prepares a potion that will cause its drinkers to fall senselessly in love forever and mixes it into both her and Tristan's wine. In Cornwall, Isolde weds Marke immediately but the potion exerts its influence on her and Tristan. They regularly meet in secret with the assistance of their servants, Brangäne and Gorvenal. Andret, a court advisor who seeks the throne, eventually exposes their tryst.
Confronted with the reality about his nephew and his wife, Marke orders their imprisonment and execution. Tristan escapes and Isolde is spared with the help of Tristan's friend Dinas. She is sentenced to live with a leper but is rescued by Tristan, and the lovers flee Cornwall. Three years later, Cornwall is under attack by the Irish, led by the traitorous Andret. Dinas finds Tristan and Isolde and urges them to return to the court and reconcile with Marke. Though Tristan is unwilling, Isolde agrees because she feels responsible for the resumption of the war. She returns to Cornwall and undergoes trial by ordeal, holding a red-hot iron to establish her innocence and restore peace.
Tristan travels to Brittany alone. He rescues a peasant woman from robbers and cynically names her "Isolde". Some time later, he is mortally wounded and sends Gorvenal to ask Isolde of Ireland to visit him on his deathbed. Gorvenal returns to Cornwall, where Andret has taken the throne. Marke learns of Gorvenal's plan and agrees to mobilise his knights to safely escort Isolde out of Cornwall. Andret is killed in the ensuing commotion. As Isolde sails to Brittany, the jealous Breton maid lies to Tristan about the color of the sails on the returning boat, implying that Gorvenal's mission was unsuccessful. He dies, and when Isolde arrives to see him, she collapses in grief and joins him in death.
