The scene is laid on board Tristan’s ship, during the crossing from Ireland to Cornwall.
Isolde is on the deck of the ship which is carrying her from Ireland to Cornwall. She is the prisoner of Tristan, who is escorting her to his uncle, King Marke, whom she is to marry. Isolde bristles with rage when she hears a sailor singing a song about an «Irish virgin», for she considers his taunting words an insult to her royal lineage. When her attendant, Brangäne, tells her that the ship is approaching Cornwall, Isolde vents her anger at her people for having yielded to the enemy in such a cowardly way.
In the second scene, Tristan is standing on the ship’s deck, moodily contemplating the sea, with his loyal squire, Kurwenal, beside him. Isolde orders Brangäne to summon Tristan to her quarters to pay his respects. Tristan refuses, and Kurwenal explains why: Tristan has heroically slain Isolde’s former betrothed, the tyrannical Irishman Morold who, after conquering Cornwall (homeland of Tristan and King Marke), had crippled the populace with high taxes. Now that Morold is dead, Tristan is taking Isolde to King Marke as a token of his victory.
Angered by Kurwenal’s insolent manner, Isolde rails against Tristan and his retinue. She tells Brangäne how a small ship, bearing a gravely wounded man, had landed on Irish shores and that, having inherited her mother’s magic potions, she was asked to take care of him. Realising that the man was Tristan, Isolde’s initial plan had been to kill him; but the look in his eyes had stirred up a mixture of attraction and pity in her heart, and she had instead nursed him back to health. His strength restored, Tristan had departed, only to return soon afterwards to convey the Irish princess to be the bride ok King Marke, a mere vassal.
After ending her tale, Isolde flares up anew at the recollection of Tristan’s unjust and arrogant behaviour, and resolves to take her revenge on him. She will invite him to drink a toast to their reconciliation – but the cup of friendship will in fact contain poison.
The ship is nearing the coast, and Kurwenal tells the women to prepare to go ashore. Isolde asks the squire to send Tristan to her, so that they may drink to the end of their strife. But Brangäne, unable to bring herself to comply with Isolde wishes, has substituted a love potion for the poison. As soon as they drink the potion, the couple are overwhelmingly drawn to each other.
Dizzy with longing, Tristan and Isolde ecstatically exchange endearments while, in the background, a loud fanfare and the jubilant cries of the crowd proclaim their arrival in Cornwall.
King Marke’s castle in Cornwall.
It is a summer’s night, and the air is full of magic. Isolde is in the garden of King Marke’s castle, awaiting Tristan’s arrival with mounting excitement. King Marke and his courtiers are out hunting, and the sound of their bugles echoes in the distance. Brangäne warns Isolde to be cautious and, above all, to beware of the treacherous Melot, who is secretly in love with her. Melot has organised the hunting party to persuade King Marke to leave the castle, knowing full well that on his return he will catch the lovers together.
Isolde brusquely dismisses her suspicion as unfounded, and extinguishes the torch burning on the castle wall, thereby signalling to Tristan that they can meet in safety. Isolde can scarcely conceal her growing desire as she impatiently awaits her lover. At long last, Tristan emerges from the dark, protective cloak of night. Falling rapturously into each other’s arms, they launch into a majestic duet, a song of praise to the night – the kingdom of nothingness and of love, contrasting with the hostile, faithless day. As they reach the height of ecstasy, Brangäne shouts from her watching place that King Marke is approaching. The courtiers, led by Melot and King Marke, burst in and discover the lovers locked in an embrace. Deeply wounded by his nephew’s betrayal, King Marke demands an explanation for his outrageous behaviour. Tristan does not answer; swept away by passion, he has banished all thoughts of the world and of his very honour, and he begs Isolde to follow him into the kingdom of night. Wildly jealous, Melot challenges Tristan to a duel, and the two men draw their swords. Tristan heroically bares his breast, into it his foe immediately thrusts his weapon.
Tristan’s castle in Brittany.
Mortally wounded, Tristan lies at the foot of a huge tree, attended by the faithful Kurwenal. A shepherd plays a melancholy strain on his pipe. Kurwenal asks the shepherd to keep watch for Isolde’s ship, and to play a cheerful melody as soon as it comes into sight. At the sound of Kurwenal’s voice, Tristan slowly regains consciousness. Lapsing into a trancelike state, he sings out in praise of the realm of night, which by now has claimed him, but revives briefly at the prospect of Isolde’s arrival. He curses the glimmering torch which keeps the lovers apart, barring them from finding eternal joy in the sphere of darkness and nothingness. Burning with fever, Tristan is convinced that Isolde’s ship is drawing towards the coast. He questions Kurwenal excitedly, only to be answered by the plaintive sound of the shepherd’s pipe. Tristan recalls hearing the same tune as a boy, when his parents died; suddenly, he is blinded by a vision of Isolde sweeping through the waves to join him. The shepherd’s pipe gives out a lively melody, announcing that Isolde’s ship is landing. Isolde rushes toward Tristan who, desperate to be reunited with his loved one, frenziedly tears the bandages from his wound. But when Isolde finally reaches him, he dies in her arms.
Brangäne appears, followed by Melot, whom Kurwenal kills. Also King Marke enters: having been informed of the exchange of potions, he intends to pardon the lovers. Instead, he is greeted with a chilling scene that fills him with despair. Isolde, no longer aware of what is happening, yearns only to join Tristan in the kingdom of night. In a prolonged final monologue, she is transfigured. As Isolde sinks lifeless over her lover’s body, a pure, peaceful note rings out, highlighting the tragic conclusion of the opera.