Tristan and Isolde at Houston Grand Opera

Tristan has left his ancestral home to serve his uncle, King Marke of Cornwall, who, childless, has made him heir to the throne. Cornwall has been forced to pay an annual tribute to the Irish king. When the Irish king's emissary, Marold, comes to claim the tribute, Tristan kills him, but in the battle he is badly wounded by Marold. It is soon discovered that Marold's sword was poisoned and Tristan's wound will not heal. The young man is set adrift in a boat that reaches Ireland, where he is discovered and nursed by the Irish princess Isolde, who had been Marold's fiancée. Tristan tries to conceal his identity, but Isolde recognizes him as her lover's slayer. She picks up the sword to avenge Marold's death but is unable to do so when Tristan gazes into her eyes. Tristan swears his gratitude and returns to Cornwall, where King Marke is being politically pressured to take a wife and produce an heir other than Tristan. Tristan offers to go back to Ireland to win Isolde's hand—not for himself, but for his uncle.

Act I

Tristan is escorting Isolde against her will to Cornwall, where she is to marry the king. On the long voyage, Isolde despairs at the prospect of a loveless marriage. Furious at Tristan for relinquishing her to another man, Isolde sends her lady-in-waiting, Brangäne, to fetch him. Tristan declines, and Isolde berates herself for not exacting revenge when she had the chance. She insists that Brangäne prepare a death potion so she and Tristan may drink it before reaching Cornwall. Just as they are nearing the shores, Tristan finally comes to Isolde. She asks him to share a drink with her, to “peace and reconciliation.” Tristan begins to drink the potion prepared by Brangäne; Isolde takes it from him and finishes it. But Brangäne has substituted a love philter for the death potion. Tristan and Isolde gaze at each other for a moment, then abandon themselves to the love they have so long concealed. They confess their love, oblivious to all else as the ship reaches the shore and King Marke approaches.

Act II

Tristan's friend Melot has planned a night hunt to keep the King occupied and give the lovers a chance to meet secretly. Brangäne suspects treachery and warns her mistress not to trust Melot, but Isolde impatiently gives the designated signal and Tristan rushes into her arms. They curse the day and invoke the night, utterly lost in each other. As daybreak approaches, Brangane warns the lovers to no avail. The King, led by Melot, arrives and discovers them. Marke sorrowfully reproaches his nephew, but not harshly enough to satisfy the vengeful Melot, who attacks Tristan. Tristan refuses to defend himself and is gravely wounded.

Act III

Kurwenal has brought his master to his ancestral home to recover, and has sent for Isolde. Finally the shepherd's pipe announces Isolde's arrival. Tristan, in an ecstatic delirium, tears open his wounds and falls dying into her arms. A second ship carrying King Marke and his retinue arrives. In fear and confusion, Tristan's servant Kurwenal kills Melot and in the ensuing melee is himself killed. But Marke (Brangäne having told him about the potion) has come not to avenge but to pardon; in fact, he has come to arrange the lovers' marriage. It is too late. Enraptured, Isolde releases her life, embracing death as the perfect union.