- (Plot summary re-constructed from other versions of the story because this portion of the manuscript has been lost)
- Rivalen, King of Lyonesse, receives the hand of Blanchefleur of Cornwall in marriage as a reward for helping her brother, King Mark, fend off an attack on his kingdom.
- Blanchefleur dies giving birth to a son, Tristan. Alert: That's the love-and-death theme, folks. It's going to keep coming.
- Rivalen has Tristan tutored in arts of war and peace by a man named Governal.
- Tristan travels to Cornwall in disguise with Governal. His skills and character soon make him a favorite with King Mark and his seneschal, Dydan.
- The brother of the Queen of Ireland, a man named Morholt, arrives in Cornwall demanding a tribute of slaves.
- Tristan asks to be knighted so that he can face Morholt in one-on-one combat in lieu of tribute, and Mark agrees.
- Tristan defeats Morholt in a fierce fight by splitting his head with his sword, a piece of which lodges in Morholt's skull. Morholt's sister saves the piece of sword, preserving it carefully.
- In the fight, Tristan receives a wound from a poisoned spear. Unable to find healing for it, he takes to the sea in a rudderless boat in the hope that God will steer him to someone who can help him.
- Tristan lands on the coast of Ireland (God has a sense of humor) and pretends to be a minstrel named Tantris.
- Tristan's harp-playing attracts the attention of Ireland's king, whose daughter, Yseut, is able to heal his wound. Tristan returns to Cornwall.
- Mark's barons are all like, "Dude, you're the king, life is good, you've got it all. But there's one thing you don't have, and that's a wife. What are you going to do about it?" The King declares that he will marry the woman whose hair falls from a swallow's beak in front of him. Yeah, we don't recommend this as a dating strategy for anyone but medieval kings.
- Tristan sets out with a few companions to find the lady with bird-food hair. They land in Ireland (uh oh), where the king has promised his daughter in marriage (UH OH) to the man who can defeat a dragon that's terrorizing the land. You see where this is going.
- Tristan defeats the dragon but is rendered unconscious by its poison tongue. The king's seneschal happens by and steals the head, then claims to be the slayer.
- Yseut and her mother find Tristan's body by the dragon and heal him. He reveals himself to be the Tantris Yseut healed previously.
- Yseut discovers the notch in Tristan's sword and matches it to the piece from her uncle's skull. At this point, Yseut is like, "Fool me once –" But then Tristan is like, "Yeah, if you kill me, you're going to have to marry the seneschal, because he's all strutting around saying he killed the dragon." Now there's a dilemma. Yseut decides it would be better to spare Tristan than to marry the seneschal. As we will see, this is one princess who has some ideas about what she wants in a mate.
- Yseut reveals Tristan to the Irish court, who pardon him for Morholt's death and grant him Yseut's hand in marriage. Tristan announces he will give Yseut to Mark.
- Before she departs, Yseut's mother prepares a love potion, which she gives to Yseut's maid, Brangain. She tells Brangain to give it to Mark and Yseut on their wedding night, possibly because she knows her daughter isn't that into older men.
- How competent is Brangain? Let's find out! Okay, on the way to Cornwall, Brangain goes ahead and just gives the potion to Tristan and Yseut by mistake. (Mistake or "mistake" – who's to say?) They declare their love and consummate it. Potion 1, Brangain 0.
- As their ship approaches Cornwall, Tristan and Yseut beg Brangain to take Yseut's place in Mark's bed on the wedding night, and she agrees. Potion 2, Brangain 0.
- Mark treats Yseut well. She and Tristan continue to meet in secret.
- Yseut attempts to have Brangain killed, since Brangain is the only one who knows about her and Tristan's affair. Potion 3, Brangain 0.
- Brangain does not reveal the affair to the men Yseut sends to kill her. When Yseut learns that Brangain didn't rat her out, she feels really bad about trying to kill her. The two make up.
- A strange knight so pleases Mark with his rote-playing that he offers him anything he wants as a reward. The knight chooses Yseut. This is getting embarrassing, Mark.
- Tristan rides to rescue Yseut. He charms the knight with his harp-playing, then carries Yseut back to Cornwall on horseback. He warns Mark to take better care of her. Now there's some irony for you, folks.