Eager to free Cornwall from the tribute it pays to Ireland, King Marke charged his vassal and nephew Tristan to face the Irish knight Morold, who comes every year to collect the tax. During the fight, Tristan manages to kill Morold but is wounded by his opponent's poison-coated sword. Only Isolde, Morold's fiancée and the daughter of the King of Ireland, is able to heal this wound, as she is the sole holder of the antidote. Under the name of Tantris, Tristan, therefore, goes incognito to Ireland. However, Isolde recognizes in Tantris the murderer of her fiancé; but something in her eyes makes her fall in love with him and gives up revenge. She heals Tristan and lets him return to Cornwall. Soon after, Tristan goes back to Ireland to bring back Isolde, who is to marry King Marke?
Act I
At sea, on the deck of Tristan's ship during the crossing from Ireland to Cornwall. Isolde expresses her anger: she is furious at Tristan's betrayal and at having to enter into a political marriage, without love, with Marke. She has Tristan sent for her next daughter Brangäne. But he declines the invitation, supported by his squire Kurwenal, who sings a song that makes fun of Morold. This only exasperates the fury of Isolde, who reveals the truth about Tristan to Brangäne. She sends her confidant to insist that Tristan speak to her before they arrive in Cornwall. Isolde is determined to wash away the insult Tristan has done to her by arousing her love and then come and get her for someone else. She asks Brangäne to prepare a poison which she finds in the box containing the potions made by her mother. When Tristan finally appears, Isolde demands reparation. Tristan offers her his sword to kill him, but she prefers to erase the affront with a ‘reconciliation drink’. In the meantime, Brangäne has discreetly exchanged the poison for a love potion. Both Tristan and Isolde drink the drink and fall into each other's arms. At the same time, the ship docks in Cornwall.
Act II
In King Marke's castle in Cornwall. Marke and his retinue went hunting. It's night, and Isolde is waiting impatiently for Tristan. However, Brangäne has noticed that Melot, one of Marke's courtiers, is keeping a suspicious eye on Tristan and warns Isolde about him. But the young woman doesn't believe what the next one says and asks him to put out the torch so that Tristan knows he can safely join her. In a great duet, Tristan and Isolde celebrate their love: they curse the day and bless the night, the kingdom of true love. There, as an omen of death, all boundaries between the lovers are abolished and the perfect union can be accomplished. Brangäne is on the lookout, but like Tristan and Isolde, she is totally surprised by the irruption of Marke, whom Melot has informed. The night hunt was just an excuse to catch the lovers in the act. Marke is deeply saddened by this betrayal, which he does not understand. Tristan doesn't know what to say to him, except that he now lives in a completely different world than he does. When, in the meantime, he invites Isolde to follow him to the ‘land where the sunlight does not shine’, Melot, seized by jealousy, comes between the young men and Tristan throws himself on the drawn sword of the courtier.
Act III
Kareol, the castle of Tristan in Brittany. In Tristan’s ancestral castle, Kurwenal stands guard over his master, mortally wounded, whom he brought back to Kareol to recover. As Tristan's wound would not heal, the squire again called Isolde for help. She may arrive from Cornwall at any moment. Kurwenal has instructed a young shepherd to watch the sea and signal to him as soon as the young woman's ship is in sight. Tristan, coming out of a half-sleep, believes in his delirium to see Isolde before him and expresses the wish to enter with her into eternal night. When the shepherd announces Isolde's arrival with a joyful melody, Tristan is overjoyed; hearing her voice, he even tears off his bandages. Isolde barely has time to hug him before he passes away. She loses consciousness and collapses on his body. A second ship docks, with Marke, Brangäne and Melot on board. Kurwenal, who doesn't know that Marke has come to bring forgiveness and reunite the lovers, rushes at the newcomers with fury. He kills Melot and then collapses, mortally wounded, next to Tristan's body. Isolde then regains consciousness one last time before following Tristan in the ‘love death’.