The action of the opera takes place in Ireland, Cornwall, and Brittany.
Before the opera begins
The Cornish knight Tristan killed in fair combat Morold, the betrothed of the Irish Princess Isolde. Isolde had the opportunity to destroy Tristan when she was deceived into healing him, but she did not because she had fallen in love with him. He left her, but returned shortly to ask her hand in marriage—not for himself, but for his childless uncle Marke, the King of Cornwall. (By finding Marke a bride, Tristan hoped to quiet rumors that he was plotting to usurp Marke's throne.) To end the feud with Cornwall, Isolde's parents gladly agreed to have their daughter marry King Marke.
Act 1
Tristan is bringing Isolde from Ireland to his uncle in Cornwall. Tristan can't explain his actions to Isolde and has refused to speak to her. Her rage knows no bounds. She tells her attendant Brangäne of Tristan's treachery and then sends word that she will only allow him to present her to Marke if Tristan comes to see her now. She then orders Brangäne to prepare a death potion for Tristan and her. When Tristan comes, he senses the drink is poisonous and welcomes it. As the two wait for the poison to take effect, they confess their love with abandon. A confused Isolde asks Brangäne what the potion was. Brangäne replies that she substituted a love potion for the death potion. Stunned, Isolde faints as King Marke boards the ship. Tristan despairs.
Act 2
At King Marke's castle in the wintry forest, Isolde begs Brangäne to extinguish the torch, whose light is a signal warning Tristan that it is not safe to approach. Brangäne warns Isolde that Marke's hunt that night is a trap to catch Tristan with her. Isolde disregards her warning. She and Tristan greet each other ecstatically before slipping into a discussion of Night, the world of passion and happiness, and Day, in which their responsibilities separate them. Brangäne warns them of the advent of day. Just as the lovers' passion rises to climax, Marke, Tristan's supposed friend Melot, and the other hunters find Tristan and Isolde in each other's arms. Neither of the two tells Marke what had happened before, even though the King begs Tristan to explain. Instead of answering, Tristan asks Isolde if she will follow him into the world of Night. She agrees; Tristan challenges Melot, then to go into Night, he falls on Melot's sword.
Act 3
Several months have passed. Tristan lies wounded at Kareol, his family home in Brittany, where his loyal servant Kurwenal has brought him. When Tristan awakens, he tells of how Isolde summoned him back to earth; when he finds out that Kurwenal has sent for her, he is ecstatic. He imagines her arriving and collapses. He curses the drink that kept Isolde and him from dying and then sees her coming across the sea to him. Before she can reach him, Tristan pulls off his bandages in ecstasy. Isolde hears him calling out her name as he dies. Marke and his retinue arrive. Kurwenal mistakenly construes Marke's approach as vengeful, kills Melot, and dies defending Tristan's body. Marke laments the death all around him. Isolde hears none of this. She describes the ascent of Tristan's soul and the waves sweeping around her. Transfigured, she joins Tristan.