For a three-act plot analysis, put on your screenwriter’s hat. Moviemakers know the formula well: at the end of Act One, the main character is drawn in completely to a conflict. During Act Two, she is farthest away from her goals. At the end of Act Three, the story is resolved.
Act I
Tristan of Lyonesse arrives at the court of his uncle, King Mark of Cornwall. He receives a festering wound while acting as Mark's champion against Morholt of Ireland and receives healing at the hands of Yseut, princess of Ireland. Later, Tristan returns to Ireland and defeats a dragon, winning Yseut for Mark. On the way back to Cornwall, he and Yseut drink a magic potion that causes them to fall in love.
Act II
Tristan and Yseut carry on their affair in secret until Tristan's blood in Yseut's bed betrays their affair. On the way to be burnt, Tristan escapes his captors and rescues Yseut, fleeing to the forest. The two live there for two years until the waning of the potion's effects causes them to regret what they have done. Yseut returns to King Mark. Tristan and Yseut again carry on their affair in secret until their discovery by Mark's barons prompts Tristan to flee to Brittany. In Brittany, Tristan marries Yseut of the White Hands but refuses to consummate their marriage. He makes a trip to Cornwall to visit Yseut.
Act III
Tristan receives a poisonous wound while helping his friend Kaherdin carry on an adulterous affair. He sends a messenger for Yseut the Fair, knowing she is the only one who can heal him. The messenger is supposed to raise white sails if he returns with Yseut. But when his ship is in sight, Tristan cannot leave his bed and asks his wife to tell him the color of the sails. Out of jealousy, she lies, saying they are black. Tristan dies in despair. When Yseut the Fair finds his body, she dies in his arms. King Mark gives them an honorable burial in a Cornish chapel, where two intertwining trees grow up over their graves.