Tristan, a valiant Cornish knight, is bringing Isolda, princess of Ireland, over as a bride for his uncle, King Mark. He is himself in love with her, but owing to a blood feud between them, forces himself to conceal his passion. Isolda, in anger at his seeming unkindness, attempts to poison herself and him, but her attendant, Brangæna, changes the draft for a love potion, which enflames their passion beyond power of restraint...