Tristan is forcing the Irish princess Isolde to marry his uncle, King Mark of Cornwall. Tristan had killed her lover Morold and tricked her into healing his wounds after the fact. She asks Brangäne, her attendant, to prepare a death potion for Tristan.

Brangäne prepares instead a love potion. Both Tristan and Isolde drink it and immediately fall in love. They meet secretly but are betrayed by Tristan's friend Melot, to King Mark. Tristan is mortally wounded in aan arugument wants to die with Isolde. Isolde joins him in death.

Brief summary of "Tristan und Isolde"
by Wagner