Tristan is escorting Isolde and her servant Brangäne by sea to Cornwall, where Isolde is to marry King Marke. The two have met before: Tristan killed Isolde’s fiancée in war, and Isolde, not knowing this, had nursed the wounded Tristan. Now Isolde knows that her escort is her fiancée’s killer, and she desires revenge. Brangäne prepares a poisoned drink that Isolde gives to Tristan as a peace offering. Brangäne, however, has replaced the poison with a love potion, and Isolde and Tristan fall in love.
Marke and his retinue are out hunting, and in the meantime Isolde meets Tristan. Brangäne warns them, as she has noticed that Tristan’s friend Melot is watching them. Tristan and Isolde pledge their love to one another and their desire to die together. Marke’s retinue arrives. Melot attacks Tristan and wounds him.
Tristan, lying wounded in the castle, sends word to Isolde, who he thinks might heal him. Isolde arrives and just has time to embrace Tristan before he dies. Marke soon enters too, realising that Tristan and Isolde were meant for each other. Unaware of the reconciliation, Tristan’s friend Kurwenal kills Melot. Isolde sees Tristan smiling at her and dies too in order to rejoin her lover.