Some time ago. Tristan’s ship at sea. Dawn.
Tristan, Knight of Cornwall, is escorting Isolde, Princess of Ireland, to Cornwall where she is being forced to marry King Marke.
A sailor sings a mocking song that offends Isolde. When Brangäne, her faithful servant, spots the Cornish coast, Isolde is overcome by rage. She casts a spell upon the ship, summoning the sea and death to rise up and to devour all on board. Brangäne tries to calm Isolde’s rage, and Isolde demands that Brangäne bring Tristan before her – she will not step ashore unless he drinks a cup of atonement with her.
Tristan evades Isolde’s request based on custom: the bridal escort remains apart from the bride on a bridal journey. Brangäne pushes Tristan to obey until Kurwenal, Tristan’s loyal servant, refuses on his master’s behalf and begins to brag about Tristan’s slaying of Morold, Isolde’s intended fiancé. As Kurwenal continues to insult Brangäne she flees the men’s chamber.
Brangäne returns to Isolde, who struggles to contain her anger at such disrespect. Isolde finally shares the truth with Brangäne: Tristan, badly wounded after killing Morold, landed on her shore under the false name of Tantris. As she began to nurse his wound with her renowned powers of healing, inherited from her mother, she quickly saw through his disguise and recognised him for who he was – the murderer of her beloved fiancé Morold. As she raised his sword to take vengeance and murder him, their eyes met, and love was born instead. Now, she is being ushered to a fate she does not want and cannot believe he wants either. Tristan’s silence and duty are destroying her heart. She asks Brangäne to help her murder Tristan for revenge. Brangäne reminds Isolde that they have brought her mother’s magic potions with them. She could use the love draught to bind her and King Marke. But Isolde has something else in mind: a death potion for herself and Tristan.
The crew prepare the ship for arrival. Isolde refuses to come ashore unless Tristan appears before her to drink atonement. Kurwenal agrees to summon Tristan and Brangäne agrees to prepare the death drink. Tristan and Isolde finally meet face to face.
Isolde demands vengeance for Morold’s death. Tristan offers Isolde his sword to slay him there, but she beckons Brangäne to bring the drink of atonement. Isolde offers it to Tristan, and, aware of something much deeper inside his own heart, he drinks it. Isolde drinks the potion as well. But Brangäne has exchanged the death draught for a love potion. As the ship reaches Cornwall, Tristan and Isolde, in the belief that they are about to die, declare their love for one another and embrace, just as Melot and King Marke arrive.
A garden within King Marke’s castle, soon after the wedding of Isolde to King Marke. Black night.
King Marke and his companions are away on a surprise night-time hunt. Although their hunting horns can be heard in the distance, Isolde is oblivious to them: she thinks only of Tristan. Brangäne warns her to be suspicious of Melot, who, she fears, has organised the hunt as a trap for the lovers. Isolde dismisses Brangäne’s warning and orders her to extinguish the light – the signal that it is safe for Tristan to come to her. Brangäne tries to stop her, but Isolde is entranced by love. Impatient to be with Tristan, Isolde puts out the light herself and sends Brangäne to stand guard.
Tristan arrives; he and Isolde greet each other ecstatically. Tristan and Isolde lose themselves in an ecstasy of love: every touch is electric. But their bliss cannot be sustained by the sunlight of day where the presence of the court and King prohibit their love. Night is their only refuge. But slowly, they realise that even night limits their union as the inevitable rising of the sun means they must once again part.
They come to the realisation that their love can only be fully united in the eternal darkness of death. And so they vow to pursue death together just as Brangäne screams out: the hunting party has returned and Melot’s trap has been sprung. King Marke confronts Tristan and asks him to account for his betrayal. Tristan is unable to explain his actions and asks Isolde to follow him into death. He accuses Melot of treachery and challenges him to fight. Tristan offers no defence and deliberately impales himself on Melot’s sword.
Tristan at his father’s estate, some time later.
Tristan lies unconscious, with his ever-faithful servant Kurwenal caring for him. An ominous shepherd’s pipe can be heard in the distance. When the shepherd appears, Kurwenal asks him to play a more cheerful melody should Isolde’s ship appear on the horizon. Gradually, Tristan regains consciousness. Delirious with pain, he thinks only of his reunion with Isolde.
Kurwenal explains that he has sent for Isolde, and in a hallucination, Tristan sees her vessel coming towards him. Hearing the shepherd’s pipe, Tristan recollects his parent’s deaths when he was a young boy, and feels the melody now calling him. He laments the potion which he and Isolde brewed and the madness he now lives – ever-waiting for Isolde to arrive and escort him to death. He deliriously imagines Isolde coming to him across the water. Kurwenal sees nothing. However, a change in the shepherd’s melody confirms that Isolde’s ship has been sighted. Kurwenal goes to look for himself and sees that Isolde is indeed arriving. Tristan throws Kurwenal out to go and greet her, and explodes with deadly passion for her.
Tristan sees Isolde and rushes to her, only to die in her arms. Isolde tries to draw one last breath from Tristan but is only able to follow him into death herself.
Kurwenal’s heart breaks seeing his dead master. The shepherd announces the arrival of a second ship: it contains King Marke, Melot and soldiers, as well as Brangaene. Thinking they have come in pursuit of Isolde, Kurwenal charges at Melot and kills him. As King Marke looks on in horror, Kurwenal rushes into death. Having learned from Brangäne about the love potion, King Marke has come to unite Isolde and Tristan and yield his own claim to her. All is dead. Isolde and Tristan transfigure through death.