There are two main traditions of th

There are two main traditions of the Tristan legend. The early tradition comprised the French romances of two poets from the second half of the twelfth century, Thomas of Britain and Béroul. Their sources could be traced back to the original, archetypal Celtic romance. Later traditions come from the Prose Tristan (c. 1240), which was markedly different from the earlier tales written by Thomas and Béroul. The Prose Tristan became the common medieval tale of Tristan and Iseult that would provide the background for the writings of Sir Thomas Malory, the English author, who wrote Le Morte d'Arthur (c. 1469).

The story and character of Tristan vary from poet to poet. Even the spelling of his name varies a great deal, although "Tristan" is the most popular spelling. Most versions of the Tristan story follow the same general outline.

After defeating the Irish knight Morholt, Tristan goes to Ireland to bring back the fair Iseult for his uncle King Mark to marry. Along the way, they ingest a love potion which causes the pair to fall madly in love. In the courtly version, the potion's effects last a lifetime; in the common versions, the potion's effects wane after three years. In some versions, they ingest the potion accidentally; in others, the potion's maker instructs Iseult to share it with Mark, but she deliberately gives it to Tristan instead. Although Iseult marries Mark, she and Tristan are forced by the potion to seek one another as lovers. While the typical noble Arthurian character would be shamed by such an act, the love potion that controls them frees Tristan and Iseult from responsibility. The king's advisors repeatedly endeavour to have the pair tried for adultery, but the couple continually use trickery to preserve their façade of innocence. In Béroul's version, the love potion eventually wears off, and the two lovers are free to make their own choice whether to cease their adulterous relationship or continue.

As with the Arthur-Lancelot-Guinevere love triangle, Tristan, King Mark, and Iseult all hold love for each other. Tristan honors, respects, and loves King Mark as his mentor and adopted father; Iseult is grateful that Mark is kind to her; and Mark loves Tristan as his son, Iseult as a wife. But every night, each has horrible dreams about the future. Tristan's uncle eventually learns of the affair and seeks to entrap his nephew and his bride. Also present is the endangerment of a fragile kingdom, the cessation of war between Ireland and Cornwall. Mark acquires what seems proof of their guilt and resolves to punish them: Tristan by hanging and Iseult by burning at the stake, later lodging her in a lazar house (a leper colony). Tristan escapes on his way to the gallows. He makes a miraculous leap from a chapel and rescues Iseult. The lovers escape into the forest of Morrois and take shelter there until discovered by Mark. They make peace with Mark after Tristan's agreement to return Iseult to Mark and leave the country. Tristan then travels to Brittany, where he marries (for her name and her beauty) Iseult of the White Hands, daughter of Hoel of Brittany and sister of Sir Kahedin.

In the Prose Tristan and works derived from it, Tristan is mortally wounded by Mark, who treacherously strikes Tristan with a poisoned lance while the latter is playing a harp for Iseult. The poetic versions of the Tristan legend offer a very different account of the hero's death. According to Thomas' version, Tristan was wounded by a poison lance while attempting to rescue a young woman from six knights. Tristan sends his friend Kahedin to find Iseult, the only person who can heal him. Tristan tells Kahedin to sail back with white sails if he is bringing Iseult, and black sails if he is not. Iseult agrees to return to Tristan with Kahedin, but Tristan's jealous wife, Iseult of the White Hands, lies to Tristan about the colour of the sails. Tristan dies of grief, thinking that Iseult has betrayed him, and Iseult dies swooning over his corpse. Several versions of the Prose Tristan include the traditional account of Tristan's death found in the poetic versions. In some sources it states that two trees (hazel and honeysuckle) grow out of their graves and intertwine their branches so that they cannot be parted by any means. It was said that King Mark tried to have the branches cut three separate times, and each time, the branches grew back and intertwined. Thereafter, he gave up and let them grow.

A few later stories record that the lovers had a number of children. In some stories they produced a son and a daughter they named after themselves; these children survived their parents and had adventures of their own. In the romance Ysaie the Sad, the eponymous hero is the son of Tristan and Iseult; he becomes involved with the fay-king Oberon and marries a girl named Martha, who bears him a son named Mark.