The story itself is a confusing composite of varied elements from different regions. The central theme may originate in Wales where the couple are called Drystan and Essyllt in the triads. The setting is oddly primitive and they steal pigs among other things. Other influences are the tale of Diarmaid and Grainne from Ireland, legends surrounding an historical Drust, a Pictish king, and various motifs of farther origin. Finally the story is placed in Cornwall and certain details betray a familiarity with the geography of the region. Mark is the king of Cornwall and Tristan is his nephew. Iseult is Mark's young wife. Tristan's home is Lyonesse, possibly a name for Lothian in Scotland at one time, but here meaning a land lying southwest of the Cornish peninsula which local legend says is now sunk under the sea.
The Cornish location has long been associated with the Tristan Stone. This 7-foot stone monument is inscribed Drustans hic iacet Cunomori filius, "Drustanus lies here, son of Cunomorus." It stands near the Cornish town of Fowey though it's present location is further from Castle Dore than originally. Cunomorus was the actual early 6th century ruler Cynvawr. By at least the 9th century and by Nennius, Cynvawr and Mark were identified as one and the same, making Mark's Latin name Marcus Cunomorus. If this is all true, then the location is correct regardless of whatever additions to the story filtered in from elsewhere. Critics say that this makes Tristan a son who runs off with his step-mother but later story tellers might have altered the relationship to make the affair seem less dishonorable. In addition, the Welsh triad "The Three Peers of Arthur's Court" also makes Tristan the son of Mark.
It is the Romancers who put the story directly into the Arthurian cycle, where it had previously been on the periphery and only loosely tied to Arthur. Tristan becomes a Knight of the Round Table and close friend to Lancelot. Though he fits in at court and has many exploits similar to other knights, he is different than them in his versatility and varied skills; he is a linguist, master harper, hunter and hawksman as well as an accomplished chess player. His abilities as a knight and warrior are great but the overriding factor of his existence is his love for Iseult and this is one of the earliest literary instances of an all consuming passion that denies all other concerns and justifies almost any action.
Tristan's adventures vary greatly depending on the author but it roughly goes like this: Tristan slays Morholt, brother to the Queen of Ireland, who has come to Cornwall to demand tribute. Once in Ireland, Tristan is discovered but his deeds and feats satisfy the Irish royals and Tristan stays with them for a period. Iseult is the daughter of the Irish king and she uses her healing skill to cure a wounded Tristan. She agrees to marry his uncle Mark but on the way to Cornwall, both she and Tristan accidentally drink a love-potion that was meant for Mark and Iseult and their doomed love is born. Mark sometimes pursues them and at other times backs away but ultimately Tristan is banished to Brittany where he marries another Iseult, Iseult "of the White Hands". However, his love for the first Iseult never dies and out of loyalty to the Irish princess he is deterred from consummating his new relationship. In verse romances, Tristan's death is due to a lie told by his jealous Breton wife: when Tristan is badly wounded, Iseult is sent for and she sails over from Cornwall. The jealous wife however tells Tristan she has not come and he dies of despair. In prose versions it is Mark who is directly responsible for killing Tristan in a cowardly attack. This causes Iseult to die of grief. When the lovers are laid to rest side by side, a vine growing from Tristan's grave intertwines with a rose growing from Iseult's.