Background
Tristan, a Breton nobleman orphaned at an early age, leaves Kareol, his ancestral home, to serve his uncle, King Marke of Cornwall. Cornwall has been forced to pay to the Irish king an annual tribute: copper the first year; silver the second year; gold the third; and young lads and maidens the fourth. When the Irish king's emissary, Morald (fiancé to the Irish princess Isolde) comes to claim the youths as tribute, Tristan slays him, but in the battle, he is himself wounded by Morald. Tristan sends Morald's severed head, with a splinter from Tristan's sword, back to Ireland as "tribute."

King Marke adopts Tristan as his heir, but it soon becomes apparent that Morald's sword was poisoned, and Tristan's wound cannot be healed. Tristan is set off in a small boat which drifts to the shores of Ireland, where he is discovered by Isolde, who nurses him back to health. Realizing his danger amidst the Irish, Tristan assumes the name "Tantris" so as not to be discovered. As Isolde tends the sleeping Tristan, she notices that the sword splinter she has kept from Morald's head matches Tristan's sword. As she picks up the sword to avenge her betrothed, Tristan awakens and looks at her - "not at the sword, not at my hand; he looked into my eyes" - and she lets the sword fall from her hands. Tristan swears "a thousand oaths of eternal gratitude and loyalty" to Isolde and returns to Cornwall.

Upon Tristan's return, the jealous Cornish barons insist that King Marke take a wife in order to provide an heir to the throne. Marke refuses until Tristan, putting honor before inheritance, threatens to leave the court if the King does not take a wife. Reluctantly, Marke acquiesces and Tristan returns to Ireland to win the hand of Isolde. Tristan performs several deeds and wins her hand in marriage. Isolde believes that she has been won by Tristan, with whom she is already in love, but only later discovers that he has won her for his king.

Act I
On the ship sailing from Ireland to Cornwall to deliver Isolde to King Marke. Isolde, not having seen Tristan the entire voyage, sends her maid Brangäne to demand that he come to her: "Let fear of me command this stubborn man!" To Brangäne, Tristan diplomatically declines, but his companion Kurwenal mockingly tells how Tristan killed the Irish hero Morald and cannot be at the beck and call of an Irish princess. Dismayed, Brangäne returns to Isolde who then recounts the entire story of her shame. Brangäne tries to calm her with assurances that King Marke is a good man, and also that Isolde's mother supplied her with a love potion for the wedding night. Instead, Isolde reveals a death potion and tells Brangäne to mix it so that Tristan can drink "atonement" for Morald's death, intent that they both shall die. Tristan arrives and, comprehending her real intention but unable to express his own passion, joins her in drinking from the cup. However, Brangäne has substituted the love potion for the poison, and as the ship arrives in port, Tristan and Isolde find their love for each other has been released. Instead of joining each other in death, they are condemned to a life of impermissible love.

Act II
The setting is King Marke's garden. Tristan's best friend Melot has arranged a hunt at night, seemingly to give the lovers an opportunity to meet. Brangäne vainly warns Isolde that Melot is treacherous and wants the throne for himself, but Isolde is impatient and signals her lover to appear. The two are reunited. They curse the "spiteful day" that has bedazzled them, and praise the night, the symbol of a death that will ensure their everlasting love. They are lost in each other even as day breaks and the two lovers are discovered by the returning King Marke and his hunting party. King Marke reproaches Tristan for having broken his oath of loyalty. Tristan yearns again for the night and asks Isolde if she will follow him. As Tristan bends down to kiss Isolde, Melot draws his sword. Instead of defending himself, Tristan lets his guard fall and is struck down.

Act III
Kareol, Tristan's neglected ancestral home in Brittany. The sound of a melancholy tune, played by an old shepherd, wakes Tristan from unconsciousness. Kurwenal has brought the mortally wounded hero home to recover and has sent for Isolde to heal him. Tristan longs for Isolde and an end to his torment. With the joyous piping of the shepherd announcing the arrival of Isolde's ship, Tristan tears off his bandages and rises to greet his lover only to die in her arms. A second ship is spotted and King Marke, Brangäne and Melot arrive with the King's pardon. Brangäne has explained about the love potion to King Marke. In a rage, Kurwenal kills Melot, and is himself slain. Isolde's lament for Tristan brings about her transfiguration as she sinks, unconscious, into the supreme bliss of death.