The story starts with the courtship of Tristan's parents. Riwalin, King of Parmenie, travels to the court of King Marke in Cornwall, where he and Marke's sister, Blanschefleur, fall in love. Blanschefleur becomes pregnant and the couple steal back to Parmenie, but Riwalin is killed in battle. When she hears the news, Blanschefleur dies, but the baby is delivered and survives. He is named Tristan because of the sorrowful circumstances of his birth.
Tristan grows up in Parmenie, passed off as the son of Riwalin's marshal Rual li Fointeant, becoming the perfect courtier. While on board a merchant ship which has docked in Parmenie, Tristan is abducted by the Norwegian crew. Once at sea, the ship is struck by a tempest, the crew conclude that they are being punished by God for abducting Tristan, so they set him ashore in a country that turns out to be Cornwall.
Tristan encounters a hunting party, whom he astonishes with his skill, and he accompanies them to Marke's court, where his many accomplishments make him popular, particularly with Marke. Eventually, after years of searching, Rual comes to Cornwall and finds Tristan, who is now revealed as Marke's nephew. Tristan is knighted.
Cornwall is being forced to pay tribute to the King of Ireland, Gurmun, collected by his brother, the monstrous Morold. Tristan challenges Morold to a duel and defeats him, though in the process he is wounded by Morold's poisoned sword. In order to seek a cure Tristan travels to Ireland incognito (under the name Tantris), and contrives to get himself cured by Gurmun's Queen Isolde (Isolde the Wise). He is struck by the beauty and accomplishments of her daughter, Isolde the Fair, and returns to Cornwall singing her praises.
Jealous of Tristan, Marke's councillors press him to marry, so that Tristan can be ousted as heir. Hoping that he will be killed in the process, they suggest Tristan be sent to Ireland to woo Isolde for Marke. Tristan travels to Ireland (as Tantris) and kills a dragon which has been threatening the countryside, thus winning Isolde's hand. However, observing that the splinter previously found in Morold's skull matches Tantris's sword, Isolde realises Tantris is in fact Tristan, and threatens to kill him as he sits in the bath. Her mother and her kinswoman Brangaene intervene and Tristan explains the purpose of his journey, which leads to a reconciliation between Ireland and Cornwall. Tristan leaves for Cornwall with Isolde as a bride for Marke.
Isolde the Wise has given Brangaene a magic potion to be drunk by Marke and Isolde on their wedding night to ensure their love. On the voyage, however, it is drunk by Tristan and Isolde by mistake. They avow their love for each other, but know that it cannot be made public, and they enjoy a brief idyll on board before arriving in Cornwall. This is followed by a series of intrigues in which the lovers attempt to dupe Marke, starting with the wedding night, when the virgin Brangaene substitutes for Isolde in the marriage bed. Marke is suspicious but is constantly outwitted by the lovers' guile.
Eventually, Marke resigns himself to their love and banishes them from court. They go off into the wilderness, to a Love Grotto, where they enjoy an idyllic life away from society. By accident, Marke discovers the grotto and sees them lying side by side. However, aware of his approach, Tristan has placed his sword between himself and Isolde, duping Marke into believing that perhaps they are not lovers after all.
With their secret hideaway discovered, the lovers return to court. However, Marke's suspicions return and finally he finds them together and can no longer doubt their adultery. Tristan flees to Normandy, where he encounters Isolde of the White Hands, daughter of the Duke of Arundel. Gottfried's poem ends with Tristan expressing his emotional confusion over the two Isoldes.
In Thomas's poem, which is preserved from around this point, Tristan marries Isolde of the White Hands, though the marriage is never consummated. Tristan creates a hall of statues, with statues of Isolde and Brangaene. Tristan is wounded with a poisoned spear by Estult li Orgillus, and sends for Isolde the Fair, who is the only one who can cure him. It is agreed that the ship sent for her will bear a white sail if it returns with her on board, but a black sail if not. However, the jealous Isolde of the White Hands lies about the colour of the sail, and Isolde the Fair arrives to find Tristan dead of grief. She kisses him and dies.
