The story of Tristan and Isolt could be reconstructed in the following way:
First, a kind of introduction which, in the Tristan of Thomas, sets the tone of the whole romance. It deals with Tristan's childhood, and his presentation as a young, poor orphan to Marc, King of Cornwall.
In a second sequence, Isolt is presented, and her relationship with Tristan is explained before the drinking of the love potion.
In an opening scene, the giant Morhot comes to Cornwall to collect the tribute of young people which this country owes to Ireland. Tristan challenges Morhot and kills him in single combat; a chip of the edge of the sword remains, however, embedded in the giant's skull.
Tristan himself receives a wound that becomes dangerously infected. No one can help him.
In accordance with the Celtic funeral rites he is put into a boat without sails and left to the currents.
The boat drifts to the shores of Ireland. Tristan calls himself a minstrel and hides his identity under a disguised name; the magic drinks of Queen Isolt, sister of Morhot, who is assisted by her daughter, Isolt the Fair, cure him. Tristan leaves his benefactresses without having revealed his identity nor spoken of Morhot and returns to Cornwall.
The second scene describes the reception of Tristan (presumed dead) by the people of Cornwall. Marc wants to make him heir to the throne in spite of the jealousy of the barons who press the King to get married. At the moment, there is nothing between Tristan and Isolt which might, in the least, resemble love.
One day a swallow drops a strand of golden hair onto the King's sleeve. Tristan remembers the beauty of Isolt the Fair, and sets sail for Ireland in a magnificently adorned ship, his mission being to obtain the hand of the blond Princess for King Marc.
But a dragon is devastating the country. The king has promised his daughter to whoever kills the monster. Tristan massacres the beast and cuts out its tongue as proof; but the poisoned tongue brings him down near the scene of the combat. Queen Isolt and her daughter Isolt the Fair find Tristan dying, take him back to the Palace and cure him. Meanwhile the Seneschal who would very much like to marry Isolt finds the dead dragon, cuts its head off and asks for his reward: the hand of the Princess.
One day when Tristan is naked in his bath, Isolt notices the chip in the edge of his sword, which reminds her of the segment found in her uncle's skull. She recognises the identity of the man she has been nursing, and even if hindered by her 'gentle womanhood', she tries to stab Tristan. He manages to calm her down by reminding her of the fact that it was he who killed the dragon, and therefore alone can prevent this cowardly Seneschal from marrying her; it will suffice for him to show the dragon's tongue, and claim his due: the hand of the Princess for King Marc.
Tristan and Isolt set sail for Cornwall amidst a cheerful crew, when destiny strikes.
Queen Isolt has prepared a love potion to be used by King Marc and the Princess her daughter, which she has entrusted to the lady-in-waiting Brangain. Tristan and Isolt, who become very thirsty during the crossing, drink it in error.
The third movement - the heart of the story as told by Beroul - deals with the love between Tristan and Isolt, and the one thousand and one stratagems they use to hide this love from King Marc who, eventually, discovers them together.
Tristan is taken to the stake, and Marc offers Isolt to a gang of lecherous lepers who are to use her as their prey. Tristan manages to escape, rescues Isolt and both flee into the Forest of Morois where, under persecution, they live in a total reclusion until the day when King Marc discovers the sleeping lovers separated by the bare edge of Tristan's sword. Marc accepts this as proof of their innocence and withdraws, leaving a glove as a sign of his visit.
Life in the forest is so hard that Tristan takes Isolt back to Marc's court before going himself into exile once more. The King accepts Isolt back but she must confirm her innocence in front of the barons by swearing an oath on the Holy Relics, and grasping a Red-hot Iron.
The finale tells us of the death of Tristan and Isolt._Tristan, away in Brittany, marries another Isolt: Isolt of the White Hands. While coming to the rescue of her brother, Kaherdin, Tristan receives a mortal wound which can only be healed by Isolt the Fair who has done this before. Marc generously lets her go. Tristan, high up on the cliffs, is waiting desperately for the white sail which brings him salvation and love; yet, in his delirium, he has no choice but to believe what Isolt of the White Hands tells him, that the sail is black.
All Tristan can do is die Isolt the Fair arrives, and falls lifeless onto her friend's body. Marc buries them together.
This is the story of Tristan and Isolt.