A ship at sea. Tristan is escorting Isolde to Cornwall to marry King Mark. Isolde hates him: he killed her fiancé Morold, then came to ask for her hand for his king. She orders Brangäne to prepare a poison: she wants to die together with Tristan. Brangäne substitutes the poison with the love potion. They both drink. They look at each other. There is nothing to be done.

King Mark has left for a hunting party. Tristan and Isolde meet at night in the castle garden. They sing of their love in an immense duet — the night as the only space in which to exist, the day as the enemy that separates them. Then the traitor Melot arrives with King Mark. Mark is shattered: the nephew he loved as a son, the promised bride. He does not shout. He only asks why. Tristan cannot answer. He lets himself be wounded by Melot.

Tristan's castle in Brittany. Tristan is wounded, feverish, delirious. He waits for Isolde — only she can heal him. Kurwenal has sent for her. Tristan alternates between states of ecstasy and despair. Finally the ship arrives. Tristan tears off his bandages and runs towards her. He dies in her arms before he can embrace her. King Mark also arrives — he came to forgive them. It is too late. Isolde sings her Liebestod and dies.