Under the command of Tristan, nephew of the English King Marke, a ship sails from Ireland to Cornwall. On the ship is the Irish princess Isolde, who is to be married to King Marke. Isolde is aggrieved about the way Tristan has been treating her during the journey, and tells her confidante Brangäne of their past history.
Tristan had slain Isolde’s fiancé Morold in battle, thus freeing Cornwall from having to pay tribute to Ireland, and sent Morold’s severed head to Isolde as a sign of his victory. But Tristan was also wounded by Morold with a poisoned sword, and thus had come under the pseudonym Tantris to Ireland, to let himself be healed by Isolde with antidotes. She recognized him from a nick in his sword: the missing splinter could be found in the head of the dead Morold. As she tried to kill him, their eyes met, and Isolde spared him, nursing him back to health. After returning to King Marke’s court, Tristan traveled once more to Ireland, this time to win over Isolde as a bride for the king, to finally bring peace between the two countries.
Humiliated, Isolde decides to kill Tristan and herself with a deadly potion. When Tristan finally comes to her, she demands vengeance for the death of Morold; Tristan is now ready to drink with Isolde. But Brangäne switches the poison with a love potion, which awakens an immense passion in Tristan and Isolde.
At that moment, the ship arrives in Cornwall.
King Marke has set out on a nightly hunt with his retinue. Isolde waits for Tristan in the garden in front of her quarters. Brangäne fears that Marke’s liege Melot is involved in a plot, and tries to warn Isolde, but she impatiently extinguishes the torch – the signal for Tristan to come.
Tristan and Isolde greet each other emphatically, and sink into an ecstatic delirium of love. Their longing for love in the “vast realm of universal night” transforms to a longing for death.
They do not hear Brangäne’s warning, and at the climax of their longing for love and death, Marke appears, led by Melot, and surprises the lovers. In response to Marke’s despairing lament about Tristan’s betrayal, Tristan has no answer. He reminds Isolde of their decision to die a shared death, challenges Melot to a duel, and throws himself upon Melot’s sword.
Kurwenal has taken the seriously wounded Tristan to Kareol, the home of his forefathers in Brittany. Since the wound refuses to heal, Kurwenal sends for Isolde. A shepherd keeps watch for her ship.
Tristan hallucinates Isolde’s revival, and loses himself in reminiscence.
When Isolde actually arrives, Tristan tears the bandages from the bleeding wound and dies, and Isolde sinks unconscious before his corpse. Then, a second ship arrives with King Marke and his entourage. Kurwenal challenges him, slaying Melot, and is himself fatally wounded. Marke, deeply hurt, laments the death of Tristan: he had come to marry Tristan to Isolde. Isolde sees Tristan awake in a transfigured form, sinking in “the vast wave of the world’s breath” with her. She dies before his dead body.