King Rivalen of Lyonesse marries the sister of King Mark of Cornwall, a woman named Blanchefleur who dies giving birth to a son, Tristan. When Tristan comes of age, he travels to his Uncle Mark's court, where his knightly and courtly skills quickly make the king think he's the best thing since yearly baths.
When Morholt, the brother of the Queen of Ireland, arrives in Cornwall demanding a tribute of Cornish slaves, Tristan is the only knight who dares to face him in one-on-one combat. He kills Morholt but receives a poisoned wound that no healer in Cornwall can treat. So he does the logical thing and gets into a rudderless boat with a prayer to God to take him to someone who can heal him. He lands on the shores of Ireland. There, the daughter of the King and Queen of Ireland, Yseut the Fair, heals him. Tristan returns to Cornwall.
King Mark's barons tell him that the only thing missing from his life is a wife, and he responds that he will marry the woman whose golden hair a bird drops before him one day. Tristan sets out with some companions to find this woman, landing again in Ireland, apparently the only place where there are blond people. While there, he defeats a dragon that has been terrorizing the kingdom but receives a terrible wound in the process.
Guess what happens next? Yseut the Fair—hey, Morholt's niece!—heals him again but also discovers that a missing part of his sword matches the piece that her mother found lodged in her uncle's skull. Nevertheless, the King and Queen of Ireland reconcile with Tristan after discovering his true identity, and they give him Yseut as a reward for defeating the dragon.
Tristan announces that he will give Yseut to King Mark. This must be great news for Yseut, but hey, no one asked her. Before Tristan and Yseut leave for Cornwall, the Queen creates a love potion that she instructs Yseult's maid, Brangain, to give to Yseut and Mark on their wedding night. You know, as long as they're in love… On the boat ride back to Cornwall, Brangain shows her utter incompetence by accidentally(?!) giving this potion to Tristan and Yseut, causing them to fall deeply in love. Looks like Brangain needs to GAIN a BRAIN. Anyone? No?
So, you guessed it, Tristan and Yseut immediately get it on. Yseut asks Brangain to take her place in bed on her wedding night. Brangain says okay, and Mark is fooled. Yseut later takes her place as Mark's queen. She and Tristan continue to get it on in secret.
Mark's barons become suspicious of Tristan and Yseut. They convince the King to kick Tristan out of the palace. From a nearby town, Tristan arranges secret trysts with Yseut in the palace garden. A dwarf named Frocin learns of these meetings and tells Mark to hide in a tree near their prearranged meeting place. When Tristan and Yseut approach the tree, they notice Mark's shadow and engage in an elaborate deception in which they pretend not to be lovers and lament that Mark has believed the "lies" of his barons. Convinced by this performance, Mark reconciles with Tristan.
Three of Mark's barons still have it in for Tristan and, with Frocin, arrange to catch him with the queen by spreading flour on the floor between her bed and his. Tristan sees the flour and leaps from his bed to hers—the things he does for love—but a wound of his reopens in the jump. He bleeds all over the floor and Yseut's bed. Based on this evidence, Mark condemns Tristan and Yseut to death by burning. As Mark's guards escort Tristan to the pyre, however, he manages to escape by leaping from the window of a cliff-side chapel to the seashore below. (No wounds reopen this time!) He rescues Yseut and, with her and his steward, Governal, escapes to the nearby Forest of Morrois.
In Morrois, Tristan and Yseut eat venison and sleep in bowers made of tree boughs. Their life is hard, but they don't mind because they're so deeply in love. One day, they meet a hermit named Ogrin who urges them to repent of their sinful love. They tell him that because of the potion, they can't. So the camping trip continues.
Meanwhile, Mark has offered a reward for their capture. But when Governal kills one of the barons who betrayed Tristan and Yseut while he's out hunting in the woods one day, everyone in Cornwall becomes afraid to enter the forest. Tristan and Yseut remain undiscovered until a forester finds them sleeping alone in a bower.
The forester leads Mark to the bower, but when the king discovers the lovers sleeping fully clothed with a sword between them, he becomes convinced of their innocence. He exchanges his ring for Yseut's and his sword for Tristan's as a sign to the lovers that he has been there and means them no harm. But when they wake up and discover Mark's ring and sword, they become convinced that Mark means to return with more men to capture them. They decide to flee to Wales.
After three years, the love potion, which made it possible for Tristan and Yseut to get through a life of hardship in order to be together, wears off. Both lovers immediately regret all they have sacrificed for love, and Tristan resolves to return Yseut to Mark. On Yseut's advice, he seeks the help of the hermit Ogrin, who helps Tristan write a letter to Mark declaring the lovers' innocence and asking that Mark take them back. Mark's barons advise him to accept Yseut, but to send Tristan away for a while, which he does. Rather than leave Cornwall, though, Tristan hides out in a friend's basement to make sure that Mark treats Yseut well.
At the instigation of Mark's barons, Yseut offers to clear her name of the charge of adultery in front of all Mark's household, with King Arthur—yes, the big A himself—and his household there to back her up. She arranges for Tristan to dress as a leper and carry her across a muddy bridge so that she can honestly say that she's had no man between her legs except Mark and the leper she rode over the bridge. Har har.
Arthur promises to defend Yseut's good name before anyone who disses it in the future, and he makes Mark agree not to believe his barons' bad-mouthing anymore. So things are great until, like, the next day, when Tristan and Yseut get it on again (hey, what happened to that potion wearing off?) and a spy betrays them to the three barons. Tristan kills two of them as they attempt to catch him in the queen's bedroom. He flees to Brittany to avoid capture.
Tristan helps the king of Brittany defend his lands from an invading army. He buddies up with the king's son, Kaherdin, and his daughter, Yseut of the White Hands. (You heard it right, folks: there are two Yseuts in this story.) When the king offers his daughter to Tristan in marriage, he accepts, believing he will never see Yseut the Fair again. But on his wedding night, the sight of a ring Yseut the Fair has given him reminds him of his love for her, and he refuses to consummate the marriage with his wife. You can bet Kaherdin is thrilled when he hears about that! However, after traveling to Cornwall and meeting Yseut, Kaherdin acknowledges that she is hotter than his sister and therefore the worthier love interest. (Don't get mad at us. We're just the messenger.)
Tristan gets the blues, and he gets 'em bad, the more he realizes that he may never see Yseut again. He disguises himself as a fool and travels to Cornwall, where he convinces Mark that he is a madman. After Mark goes out hunting, Tristan attempts to reveal his true identity to Yseut by recalling the events of their life together. She does not believe it is really him until he shows her the ring that she gave him, at which point she makes up for not recognizing him by making sweet love with him until Mark returns. Tristan does not stay long in Cornwall.
Back in Brittany, Tristan helps Kaherdin conduct an affair with the wife of another knight. One day when leaving a tryst, Kaherdin is killed and Tristan horribly wounded by the knight's men. Tristan knows that only Yseut the Fair can heal him, and he sends a messenger to fetch her. Upon his return, the messenger is supposed to raise a white sail if he brings Yseut with him and a black sail if he doesn't. Too weak to leave his bed when the ship returns, Tristan asks his wife to tell him the color of its sail. But she has overheard Tristan's arrangement with the messenger and, in a fit of jealousy, tells him that the sail is black when it is actually white.
Believing that his love has failed him, Tristan dies. Yseut hurries to Tristan's chamber. She kisses Tristan and dies embracing his dead body. The bodies of the lovers are transported back to Cornwall, where King Mark has decided to give them an honorable burial. He buries them on either side of the apse in the church. Two trees with intertwining branches grow up repeatedly over their graves despite Mark's attempts to curb their growth, a phenomenon people attribute to the presence of the love potion in Tristan and Yseut's bodies.