A Cornish ship is returning from Ireland to the kingdom of Cornwall. At the helm is Tristan, nephew and adopted son of the Cornish King Marke. Tristan is conducting Isolde, daughter of the king of the vanquished Irish, to Cornwall. His task is to get her to marry the widowed Marke in order to strengthen the alliance between the two peoples as well as his own position of power. On board the ship he keeps his distance from Isolde. Through Brangäne, her confidante, she invites him to pay her the respect which is due to her. Instead of this, Tristan’s trusty follower Kurwenal mocks Isolde with a scornful song in which he is joined by the whole crew. This prompts Isolde to reveal to Brangäne what happened in the past :
Tristan had in fact killed Isolde’s betrothed Morold on the battlefield. In close combat, Morold had, however, inflicted a poisoned wound on Tristan which only Isolde could heal. The ailing Tristan, disguised as the minstrel »Tantris«, had caused his boat to be stranded on the Irish coast so that he could be tended by Isolde. Isolde eventually recognised him, but was incapable of taking revenge when the sick man looked into her eyes. She nursed him back to health. »Tantris«, thus healed, later returned to Ireland under his real name to win Isolde as King Marke’s bride. The Irish, defeated in battle, had no choice but to accept this proposal.
After Brangäne has reminded Isolde of her mother’s magic potions smuggled on board, which could be used to bring about a positive outcome, Isolde, deeply hurt, resolves to poison both Tristan and herself. Her threat, not to appear before the king unless Tristan pays her a visit, causes the latter to approach. His arrogant defensiveness subsides when he realises that Isolde has seen through the double-dealing of his Tantris disguise. He joins her in the supposed »drink of atonement« which Brangäne serves, fully aware of Isolde’s intention to poison him. But instead of the poison, Brangäne has poured them the love potion. As they await their death, Tristan and Isolde declare their love for each other. At this very moment, the ship lands in Cornwall.
A summer’s night in the garden of King Marke’s castle. Isolde is now Marke’s queen. While the King and his court are out hunting, Isolde impatiently waits for Tristan to visit her in secret. Brangäne warns her, fearing that the crooked courtier Melot has set a trap for her, but Isolde will not hear of it. She extinguishes the torch, giving Tristan the signal to approach. The lovers embrace passionately. Very soon, however, Isolde speaks of Tristan’s betrayal of their love by making her marry King Marke. Tristan holds the »spiteful day« responsible that has caused him to prize order and success above all else. He praises the alternative world of the »wondrous realm of night« and its promise of the »eternal bliss of love«. And yet Isolde cannot escape the prison that is her marriage, where Tristan has locked her up. Their mutual yearning for love transmutes into a yearning for death. They do nothing to avoid being discovered by Marke and Melot. Marke’s despair is evident when confronting Tristan with the enormity of his disloyalty, to which Tristan has no answer. Instead, he thrusts himself upon Melot’s sword.
And yet Tristan has not been fatally wounded. Kurwenal, his friend, has taken him to Kareol, his family castle in Brittany, hoping against hope that some cure might be found. Only Isolde’s arrival, so Kurwenal hopes, will save his lord from dying. A shepherd promises to interrupt the melancholy tune he plays on his shawm and change to a more joyous one as soon as he sees her ship approaching.
Tristan awakes from a trance-like state. He cannot die as long as yearning for Isolde will keep him alive. He tries to make sense of his misadventures and curses his own longing, ending with an ecstatic vision of Isolde’s arrival. When the shepherd finally heralds Isolde’s approach, Tristan tears the dressing from his wounds and bleeds to death. Marke, Melot and Brangäne had been following Isolde with the intention of marrying her to Tristan. Yet Kurwenal, unaware of this, kills Melot and is in return struck down when Marke defends himself.
Isolde, kneeling beside Tristan’s body, pictures her lover’s resurrection.