Tristan was born into tragedy. His father was Rivalen, King of Lyonesse, a land said to lie beneath the sea off the coast of Cornwall, and his mother was Blancheflor, the sister of King Mark of Cornwall. Rivalen was killed in battle before Tristan was born, and Blancheflor died in childbirth, leaving the infant an orphan. King Mark took the boy into his court at Tintagel and raised him as his own son, training him in every art of knighthood. Tristan grew to be unmatched in combat, in hunting, and in music. He could play the harp so beautifully that wild beasts would gather to listen, and his voice was the sweetest in Cornwall. The Welsh Triads name Drystan, the Welsh form of Tristan, as one of the Three Enemy Subduers and one of the Three Warriors who wore a coronet in battle, marks of the highest distinction in early heroic tradition. In the earliest Arthurian tale, Culhwch and Olwen, Drystan appears at King Arthur's court at Celliwig, surnamed Hayarn, Iron-Fist, a warrior of proven strength.

The defining episode of Tristan's life began with a wound. He was struck by a poisoned sword in combat, and no healer in Cornwall could cure him. He sailed to Ireland in disguise, where the queen, also named Isolde, was famed as the only physician who could save him. She healed him, and he returned to Cornwall, but the Irish king had a condition for any future suitor: the princess Isolde, his daughter, would only marry the man who could slay the dragon that was terrorising the Irish countryside. King Mark sent Tristan to win her hand. Tristan killed the dragon, claimed the princess, and set sail for Cornwall with Isolde, her maid Brangaine, and a love potion that Isolde's mother had prepared. The potion was meant to ensure that Isolde and King Mark would love each other truly on their wedding night. But during the voyage, Tristan and Isolde drank from the wrong vessel. The potion took hold instantly, binding them together in a passion that would last for the rest of their lives, a love so powerful that all other considerations became irrelevant.

Their affair was conducted in the shadow of King Mark's court, a maze of deception, secret meetings, and near-discoveries. On one occasion, Mark found the lovers asleep in the forest of Morois with Tristan's sword laid between them, a symbol that Tristan insisted proved their innocence. Whether Mark believed this or chose to accept it out of pride, the ruse bought them time. But suspicion grew, and eventually Tristan was forced into exile. He left Cornwall and travelled to Brittany, where he entered the service of the Duke of Brittany and won great renown in combat. The duke offered him his daughter in marriage, a maiden called Isolde of the White Hands. Tristan accepted, but he could not consummate the marriage. His heart belonged to the other Isolde, the Irish princess who was now Queen of Cornwall, and no other woman could take her place.

Tristan's end came in Brittany, where he was wounded by a poisoned spear during a battle. He lay dying, and he knew that only Isolde of Ireland, the healer who had once saved him before, could cure him now. He sent his loyal companion Kaherdin across the sea to beg her to come, and he made Kaherdin promise to hoist a white sail on the return voyage if Isolde was aboard, and a black sail if she was not. Isolde came, but the voyage was slow, and Tristan lay wasting in his chamber, watching the harbour from his window. His wife, Isolde of the White Hands, jealous and vengeful, saw the white sail on the horizon and told Tristan that it was black. Believing that his true love had abandoned him, Tristan turned his face to the wall and died. When Isolde of Ireland arrived and found him dead, she lay down beside him, embraced him, and yielded up her own life in a final kiss. In some versions, she dies of grief; in others, she simply wills herself to follow him. They were buried in the same chapel, and from Tristan's grave grew a rose, while from Isolde's grew a vine, the two plants twining together so tightly that no hand could part them.

The story of Tristan and Isolde entered the Arthurian world most fully through the Prose Tristan of the 13th century, which incorporated Tristan into the Round Table fellowship and made him a companion of Lancelot and the other knights. Thomas Malory drew heavily on the Prose Tristan for his Book of Sir Tristram de Lyones in Le Morte d'Arthur, and it was Malory who gave the story its English form. But the roots of the legend are far older and more Celtic than the French romances. A memorial stone near Castle Dore in Cornwall, dating from the 6th century, bears the inscription DRVSTANVS HIC IACIT CUNOMORI FILIVS, "here lies Drystan, son of Cynfawr," and a 10th-century Anglo-Saxon charter names a hryt eselt, "Isolde's Ford," in the same landscape. The Welsh Black Book of Carmarthen preserves a fragmentary poem in which Drystan speaks of having betrayed March for Esyllt, suggesting that the love story was known in Wales by the 10th century. Whether the stone commemorates the historical figure behind the legend or whether the legend grew up around the stone, the connection between Tristan and the Cornish landscape is as old as the Arthurian tradition itself.