An awning on the upper deck of a ship, richly adorned with carpets.

Isolde, princess of Ireland, is sailing to Cornwall, where she is to be given in marriage to King Marke. She is escorted by Tristan, the King’s nephew. Isolde is resting, motionless on luxurious cushions, under a canopy that separates her from the ship’s crew. Beside her is her handmaid Brangäne, who lifts a hem of the curtain and gazes out to sea. A young sailor sings a song alluding to Isolde’s fate (“Westwärts schweift der Blick”). Struck by his words, the princess flies into a rage and orders Brangäne to bring Tristan into her presence. Tristan however refuses, politely but firmly, to abandon the helm. When Brangäne insists, the squire Kurwenal makes fun of her in a provocative song about the combat in which Tristan defeated and slew the Irish champion Morold, Isolde’s betrothed, who had demanded tributes from King Marke. While the sailors sing the refrain and join in Kurwenal’s jests, Brangäne hurries back in fear to shelter under the canopy beside her mistress.

Isolde, alone with Brangäne, tells her sad tale (“Den hab’ ich wohl vernommen”). One day in Ireland, shortly after Morold’s death, she had stopped to help a mortally wounded knight, who had told her that his name was Tantris. With her knowledge of potions and ointments, Isolde had saved him from death. But she had recognised him as the murderer of her betrothed, after noting that missing from his sword was the fragment extracted from Morold’s wound. Grasping the sword she had approached Tantris’s bed, determined to revenge herself on him. But the knight’s gaze had struck her and caused her to put down the weapon. When later Tristan had returned to ask for her hand in marriage to King Marke, in token of reconciliation between Ireland and Cornwall, Isolde had recognised him as Tantris. Now Isolde curses Tristan and his betrayal; but Brangäne guesses that in reality, concealed behind the princess’s desire for revenge, is love. So she tries to soothe Isolde and suggests that she give Tristan a love philtre to drink. Isolde instead orders her to bring the magic potions inherited from her mother, and chooses from them a deadly poison.

Suddenly Kurwenal enters the canopy, telling the two women to get ready to disembark. But Isolde demands Tristan’s presence. When he appears, she hands him a filled goblet in token of reconciliation. Tristan hesitates, but then drinks. Isolde snatches the glass from his hand and pours the remaining liquid down her throat to the last drop, having decided to die too. Brangäne however has substituted the mortal liquid with the love potion. Dropp­ing the goblet, Tristan and Isolde gaze into each other’s eyes, consumed with intense emotion. In ecstasy, they stand clasped in a long and passionate embrace, oblivious to what is going on around them. The sailors announce that the ship has landed, and that King Marke and his retinue are on their way.