Tristan defeats an Irish king and returns to his place to bring back Iseult/Isolde for his uncle so the uncle can marry her. They drink a potion to get two people to fall madly in love, and Tristan and Iseult/Isolde loved each other for 3 years (in some variations it was a lifetime). Iseult/Isolde is married to the uncle, but she badly wants Tristan. Usually in these stories they'd be ashamed, but the potion lifted them of any feelings of responsibility. The advisors for the king try to get them tried for adultery, but the couple tricks them up to avoid the charges.

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A love triangle forms, with everyone loving each other after the potion wears off. Tristan respects the King as a mentor and father, Iseult/Isolde is happy the King is nice to her, and the King loves both, Tristan as a son and Iseult/Isolde as a wife. The King has horrid dreams about the future, and discovers the Tristan-Iseult affair, and plans to trap them both. Along with this issue, the kingdom is very fragile with the threat of war looming. The King finds evidence of the affair, and sentences both to different deaths. Tristan by hanging and Iseult/Isolde by burning at the stake, later choosing to put her into a leper colony. Tristan escapes from the hanging on the gallows and rescues Iseult/Isolde by a miraculous jump into the chapel. The couple escapes into the forest and stay there until discovered by the King. The King strikes a deal where Iseult/Isolde would be with the King again, and Tristan would be forced outside of the country, later meeting and marrying a princess, (for her name and her beauty) Iseult of the White Hands, daughter of Hoel of Brittany and sister of 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 of Ireland, 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 French sources, such as those carefully picked over and then given in English by the well-sourced and best-selling Belloc translation of 1903, it is stated that a thick bramble briar grows out of Tristan's grave, growing so much that it forms a bower and roots itself into Iseult's grave. It goes on that King Mark tries to have the branches cut three separate times, and each time the branches grow back and intertwine. This behaviour of briars would have been very familiar to medieval people who worked on the land. Later tellings sweeten this aspect of the story, by having Tristan's grave grow a briar, but Iseult's grave grow a rose tree, which then intertwine with each other. Further tellings refine this aspect even more, with the two plants being said to have been hazel and honeysuckle.