To free Cornwall from paying tribute to Ireland, Tristan, nephew of King Mark of Cornwall, had killed Morold, the champion of Ireland, in single combat. Severely wounded in the battle, Tristan had made his way to Ireland where, under the assumed name of Tantris, he had been healed by the magical arts of Isolde, daughter of the king.
Isolde, who had been betrothed to Morold, recognised Tristan (a piece broken out of his sword matched the fragment in Morold's head), but spared his life. On his return to Cornwall, to satisfy jealous courtiers that he was not aspiring to the throne, Tristan had persuaded his uncle to ask for the hand of Isolde.
On board Tristan's ship returning from Ireland to Cornwall
Raging at her fate, Isolde laments that she has not her mother's magic art of calling up a storm to wreck the ship. She is angry that Tristan keeps apart and sends her attendant Brangäne to summon him. Tristan answers mildly that he must steer the ship; but his retainer Kurwenal answers roughly that Tristan is not Isolde's vassal, singing a vigorous song, taken up by the sailors and clearly audible to Isolde, celebrating Tristan's killing of Morold.
Isolde tells Brangäne how she had spared Tristan's life when he was in her power and complains bitterly that he has repaid her by seeking her as a bride for his old uncle, an insult that no one would have dared if Morold were alive and Cornwall still owed tribute to Ireland. Brangäne reminds her of the love potion which her mother has given her, but Isolde thinks only of the poison which is in the same chest. When Kurwenal announces their imminent arrival in Cornwall and tells her to prepare to land, she gives him a message for Tristan: he must make amends for an unatoned wrong. She orders Brangäne to pour the poison into a golden goblet.
When Tristan arrives, she reproaches him with having avoided her during the voyage. He replies that he intended no discourtesy, but rather greater respect by keeping his distance from his uncle's bride. Isolde claims that Morold's blood still lies between them, as she was not party to any reconciliation. He offers his sword for her to kill him if Morold was so dear to her, but she says King Mark would hold it against her if she killed him. Instead she offers a drink of atonement. Understanding her intention, he drinks and Isolde seizes the goblet and drinks the remainder.
But Brangäne has substituted the love potion and they fall into each other's arms, oblivious of all around them, as the ship reaches land and King Mark approaches to claim his bride.
A garden outside Isolde's chamber in King Mark's castle in Cornwall
Isolde waits for Tristan as King Mark and his court leave on a hunt. She brushes aside Brangäne's warning of danger, particularly from Melot, answering that he is Tristan's friend and has arranged the hunt so that Tristan can meet her. Brangäne, suffering pangs of remorse for her part in the love between Tristan and Isolde, begs her to defer the meeting. Isolde says that not Brangäne, but love itself (Frau Minne) was responsible for their love. She gives the signal by extinguishing a torch, telling Brangäne to keep watch.
Tristan arrives and the lovers embrace, praising the night as the friend of their love, as opposed to the inimical day. Night, as Tristan explains, is equivalent to death and death will not part them, but unite them forever: only in death can their love be truly fulfilled. Brangäne's warning of the approach of day goes unheeded and they are taken by surprise when Kurwenal rushes in, followed closely by King Mark, Melot and the court.
King Mark rejects Melot's claim to have saved him from shame, as nothing can remove the deep wound of Tristan's betrayal. He asks why Tristan, his dearest friend, has betrayed him, recalling that when his wife died he refused to marry again for Tristan's sake, intending him as his heir, bowing only to the wishes of his people whenTristan added his voice to theirs. Tristan had found him a peerless bride and his joy in her had made him more vulnerable than before.
Tristan replies that he is unable to offer an answer that his uncle would understand. He asks Isolde if she will follow him in death. She consents and he kisses her on the forehead, arousing the wrath of Melot, who draws his sword. Tristan draws his sword to defend himself, but lets it fall and is wounded by Melot.
Tristan's ancestral castle in Brittany
Tristan, his wound still unhealed, lies under a tree as a shepherd plays a mournful tune while keeping watch for a ship which is bringing Isolde. Tristan wakes, but is unsure where he is. Kurwenal tells him that he has been brought home to recover from his wound. Tristan replies that he has already been in death's kingdom and only returned to find Isolde, who still tarries in the realm of light.
Kurwenal explains that Isolde has been sent for, as the only one able to heal his wound, as she had done before. In delirious excitement, Tristan thinks he sees the ship approaching. Finding this is not so, he is reminded by the shepherd's mournful tune of the same song which accompanied the griefs of his childhood, when he learnt that his father had died before he was born and his mother on giving birth to him. He curses the potion, bearing both love and death, which he himself had brewed. The shepherd's tune changes to a cheerful strain -the ship has been sighted. As Kurwenal goes to the shore to meet Isolde, Tristan tears off the bandage from his wound. He is only able to murmur Isolde's name, before dying at her feet. As she reproaches him for dying before her, another ship is sighted.
Kurwenal tries to bar the gate as King Mark and Melot arrive with Brangäne. Kurwenal kills Melot and is killed by Mark's men, falling at his master's feet. The king grieves to find this carnage. He had been told of the potion by Brangäne and had not only forgiven Tristan and Isolde but had come to unite them. Isolde, oblivious of her surroundings, is transfigured as she joins Tristan in death.