The deck of a ship. The Irish princess Isolde is being carried to England aboard Tristan’s ship as a peace prize, to marry Tristan’s uncle, King Marke of Cornwall. She is hardly at peace within: Tristan killed her husband Morold in battle and then came back to her in disguise, hoping to avail himself of her healing arts for his own wounds. At the moment she recognized him and found a way to kill him, their eyes locked and she fell madly in love. Now she is bitterly resentful, and Tristan has ignored her throughout the voyage. Isolde orders Brangäne, her maid, to command Tristan to appear before her. Tristan’s squire, Kurwenal, mocks the Irish women with a song in praise of Tristan. Isolde, furious, reminds Brangäne of Morold’s death, of Tristan’s disguise, of her healing powers, of her old desire for revenge – and of her helpless passion. She asks Brangäne to prepare her mother’s death potion.

The sailors sight land. Isolde insists she will not accompany Tristan ashore until he apologizes to her. He greets her courteously, and she bids him toast to reconciliation. Knowing she intends to kill them both, he takes the goblet and drinks. Then she drains it … and they fall into each other’s arms. Brangäne has switched the drinks and given them a love-potion. She separates the ecstatic pair just in time for all to disembark.

Isolde’s apartments, opening onto King Marke’s palace gardens. Brangäne hears the hunting horns. She is sure that Tristan’s enemy Melot has arranged the expedition to trap no animal prey but Tristan and Isolde themselves. Isolde nevertheless urges her to extinguish the night torch, giving the signal for Tristan to come. Brangäne warns her again of Melot’s treachery. At last Isolde sends her to stand watch, and puts out the torch herself. Tristan enters. The lovers, enraptured, defy the approach of dawn and the warnings of Brangäne. They welcome night’s sweet redeeming oblivion. Kurwenal rushes in, begging Tristan to save himself. The King’s soldiers surround the lovers, still locked in a mad embrace. Tristan helps hide Isolde from view. King Marke, in shock, wonders how his dearest nephew, whose mission – whose idea! – was to bring Marke a bride, could so dishonor him. Tristan makes no defense. He invites Isolde to follow him to the land of night. She accepts. Tristan lets his sword drop and Melot deals Tristan a mighty blow, prevented from killing him only by the hand of Marke.

A ruined castle along the coast of Brittany. Tristan, gravely wounded, is back at his home, Kareol. Kurwenal keeps watch over him. A shepherd peers over the wall to ask how Tristan fares, and promises to play his sad tune until he sees a ship approaching, then he will change to a happy one. Tristan revives, asks Kurwenal where he is, and Kurwenal, overjoyed by the signs of life, tells him he lies in his own castle. Tristan raves of love and war, imagines he sees Isolde coming towards him. Can Kurwenal not see the approaching ship? he asks. And at last, the shepherd’s pipe plays a strain of joy, for over the horizon comes the vessel carrying Isolde. In his frenzied ardor, Tristan rips the bandages from his wound. Kurwenal goes to bring Isolde ashore and Tristan staggers toward her. No sooner have they uttered each other’s names than he dies in her arms.

A second ship arrives bearing the King and his men. Believing they have come in pursuit of Isolde, Kurwenal and his men attack. He kills Melot to avenge his friend, but is stabbed by the King’s men. He dies at Tristan’s side. Brangäne rushes in to say that King Marke comes bringing forgiveness for the potion. Isolde is already far away, lost in contemplation of her dead lover. She sinks upon his corpse in ecstasy.