Tristan und Isolde is Wagner's opera in 3 acts set in legendary Brittany and Cornwall, with a tragic love story revolving around the themes of love, night, and death.
Isolde is being brought from Ireland to Cornwall by Tristan to be the bride of his uncle, King Mark. Isolde is angry at Tristan because he slew Morold, Isolde's betrothed, who came from Ireland to exact tribute from Cornwall. However, when she sees Tristan, her feelings begin to change. She plans to poison him, but when she and Tristan drink from the same cup, it turns out that Brangäne, her maid and confidante, has filled it with a love potion.
At King Mark's castle, the young lovers meet at night for trysts, while Brangäne stands guard. Mark and his courtiers go off hunting one night, and Brangäne warns Isolde Melot, Tristan's supposed friend, has arranged the nocturnal hunt as a trap. Isolde and Tristan disregard this and sing love songs to each other. Then King Mark, Melot and the courtiers burst in on them as the sun begins to rise. Mark asks how they could do this; Tristan says he is willing to go into the realm of night, and Isolde says she will follow him there. Melot and Tristan have a swordfight, and Tristan allows himself to be wounded.
In Act 3, Tristan is dying at his castle in Brittany. His servant and friend, Kurwenal, watches over him. They watch for a ship that will bring Isolde, and at last she comes. They share a last moment of love, then he dies in her arms. A second ship arrives with King Mark, who plans to let Tristan and Isolde remain together. Kurwenal attacks them and is slain; Isolde falls on to Tristan's body, dying of grief.