In the days when Merlin was still free, King Melyodas de Lyones married the sister of King Mark of Cornwall and had a son by her, called Tristram (Tristan), "the sorrowfull-bonrne." The circumstances of his birth are vaguely analogous to those attending the birth of Arthur: the death of a parent — in this case Tristram's mother — unrest in the kingdom, aid to the throne from Merlin. Seven years later, King Melyodas marries again. His new wife, wishing that her own sons might succeed to the throne, plots to kill Tristram. When her plot is discovered, she is sentenced to burn, but Tristram himself pleads mercy for her and saves her. Melyodas sends Tristram to France, where he becomes a great huntsman and harper. At eighteen he returns to his father's hall.
Tristram distinguishes himself by killing Marhault and thus freeing King Mark of a debt of tribute owed to King Angwyssh of Ireland; but after the fight Tristram has wounds that cannot be healed except back in Ireland, where he got them. He returns, taking his harp, and is soon called to Angwyssh's court to play. He goes, converting his name to Tramtrist, and says nothing of his fight with Marhault, the queen's brother. Angwyssh's daughter Isode (Isolde) heals Tristram and they fall in love. He defeats her suitor Sir Palomydes in a tournament and forces him to abandon his suit for the moment.
But now Tristram's identity is discovered. Reluctantly, King Angwyssh expels him from court; Tristram says farewell to Isode and returns to Mark at Tentagil Castle. He serves Mark for some time, but at last Tristram and Mark fall out over a lady, the wife of Sir Segwarydes. Mark tries to murder Tristram, but fails. Soon afterward Tristram finds the lady not worth his love, as he thinks, and vows he will henceforth "beware what maner of lady I shall love or truste."
Mark, still plotting to murder Tristram, sends him after Isode, whom he intends to marry on the basis of Tristram's praise of her. Tristram's ship is driven to England in a storm, and there by chance King Angwyssh has been summoned to defend himself against an accusation of murder. Tristram offers himself as Angwyssh's champion in a trial by combat and fights Launcelot's cousin Blamoure. He beats him but refuses to kill him, and so Tristram both frees King Angwyssh and becomes a friend of Launcelot's house. King Angwyssh offers Tristram any gift he may ask, and Tristram requests Isode, not for himself but for Mark, as his mission requires.
Isode and Tristram sail for Mark's court in Cornwall. On the way, a love potion prepared for Isode and King Mark accidentally falls into the hands of Tristram and Isode, who unwittingly drink it and thus seal their unlucky love. Before reaching Cornwall Tristram has further adventures which by chance parallel adventures Launcelot is undergoing at the same time. Then the lovers arrive at court; Isode and Mark are married, but she and Tristram remain lovers.
Palomydes shows up and again Tristram fights him. Then all goes well for a time, until Andret, Tristram's cousin, jealous of Tristram's glory, shows King Mark that Tristram is talking with Isode at her window. Mark attacks, Tristram ludicrously overcomes and humiliates him — chasing the king and slapping him with the flat of his sword until the king falls on his nose. Afterward, Mark's advisers recommend that he make peace with Tristram, the best of his protectors, and Mark gloomily does so.
But Tristram's troubles are by no means over. Lamerok of Wales, partly to get revenge on Tristram for shaming him once, and partly to divert to Mark's court troubles that must otherwise come to Arthur's, sends King Mark a magical cup which can reveal whether women are loyal. When Isode fails the test, Mark's advisers tell him not to trust a mere sorcerer's cup.
Andret then lies in wait with twelve knights in Isode's bedroom and, when Tristram lies down naked beside her, leaps out and seizes him, binds him, and takes him to the king. Tristram reminds the court of all he had done for Cornwall, but Andret scoffs and prepares to kill him on the spot. Tristram breaks free, kills Andret, and escapes. After this Tristram and Isode live happily for a time in a forest hut, but at last Mark gets Isode back and imprisons her.
Tristram, again suffering a wound that will not heal — and now unable to get to Isode for help — is forced to go to Britain, to another lady, Isode le Blaunche Maynes. Time passes and at last Tristram marries the new Isode but will not consummate the marriage. He hears now that Launcelot scorns him for his falsehood to his lady. Guinevere meanwhile writes letters of comfort to La Beal Isode.