Tristan is taking Isolde, King Marke’s bride-to-be, to Cornwall by ship. Brangäne, who believes it to be a good match, is surprised by her mistress’ rage and despair. Isolde orders Brangäne to call Tristan, but he declines to offer any explanations. Kurwenal, Tristan’s loyal friend, responds arrogantly: a hero such as Tristan has nothing to discuss with a woman who is being taken in captivity to be the wife of another man. Isolde admits to Brangäne that Tristan is the very same knight who once killed her betrothed and whom she almost took revenge on but, in an instant of compassion, instead cured his dangerous wounds. He swore to be faithful to her, but now, thankless, he has abducted her for his uncle, the elderly King Marke. In despair, Isolde wants to prepare poison for herself and Tristan and asks him to be summoned so they can drink the potion of atonement together which will help him forget his guilt. This time Tristan comes. Brangäne, however, brings a love potion instead of the poison. Tristan guesses at the true intentions behind Isolde’s vague words about atonement and accepts the goblet. Expecting to die soon, Tristan and Isolde declare their love for one another. At this moment the cries of the sailors herald the ship’s arrival in Cornwall.
Isolde anxiously awaits a meeting with her beloved. Brangäne warns Isolde that Melot the knights has been watching Tristan ever since his arrival; Melot is in a position to tell the King all and so Brangäne begs Isolde to take care. Isolde, however, believes Melot to be Tristan’s friend, who has specially taken the King hunting to arrange this meeting. Tristan arrives. The lovers praise night and death, which for them are greater than the glorious light of day. They pay no heed to Brangäne. Who warns them that dawn is breaking until Kurwenal runs in shouting “Save yourself, Tristan!” But close on Kurwenal’s heels King Marke and his retinue appear. Melot denounces Tristan. The King upbraids his nephew for his ingratitude. Tristan asks Isolde if she is prepared to follow him. In indignation Melot attacks Tristan, wounding him severely.
The loyal Kurwenal has brought Tristan to his castle in Brittany. All of Tristan’s thoughts are of his birth and his childhood. Here, in Kareol, as soon as Tristan was born his father tragically died. It was here that his mother died giving birth. A shepherd’s mournful tune makes Tristan reflect on his life and admit the insurmountable lure of death, as if it was all born together with him. Kurwenal tells Tristan that he has sent for Isolde – she alone can cure his wounds. Tristan awaits Isolde with all his being and as the ship arrives he dies. Isolde cannot believe Tristan is dead. Marke appears with his retinue. Kurwenal, unaware of the purpose of their visit, wages battle against them, kills Melot and himself dies. Brangäne, who convinced Marke to come to Kareol, explains to Isolde that the King has forgiven the lovers and has come to bless their union. Isolde hears nothing. She follows after Tristan.