Most good stories start with a fundamental list of ingredients: the initial situation, conflict, complication, climax, suspense, denouement, and conclusion. Great writers sometimes shake up the recipe and add some spice.
Initial Situation
Tristan claims Yseut as a bride for his uncle and lord, King Mark.
All the ingredients for a classic love triangle are firmly in place: beautiful maiden, knight in shining armor, and a lord with a claim to the beautiful maiden. To make things even juicier, the knight in shining armor is a vassal to the lord, who also happens to be his uncle.
Conflict
Tristan and Yseut drink the love potion and fall madly in love.
Once Tristan and Yseut fall in love, they're in conflict with the rules of their society and with Yseut's future husband, who we're guessing won't look too kindly upon his wife sleeping with another man. To complicate matters, Tristan owes his undying loyalty to Mark.
Complication
Mark's barons learn about Tristan and Yseut's adultery.
The cat's out of the bag for Tristan and Yseut. Once Mark's barons learn about their adultery, they throw up all kinds of obstacles for the lovers. First, Mark forces Tristan to move out of the palace, forcing him to arrange secret meetings with Yseut. Then, they get Frocin to spread flour all over the floor between Tristan and Yseut's beds. What's a lover to do?
Climax
The barons apprehend Tristan in Yseut's bedroom. Tristan escapes from them with a spectacular leap from a seaside chapel, then rescues Yseut.
The moment you always knew was coming is here: Tristan and Yseut get caught. Their capture, and the proof of their adultery, is the culmination of the events that were set in motion when they drank the love potion. Their fate was always leading them to this.
Suspense
Tristan and Yseut live a life of exile in the forest of Morrois.
Tristan and Yseut's forest life is pretty much one long moment of suspense. On several occasions, like when one of the barons goes hunting near their hiding spot, or a forester discovers them sleeping in a bower and leads Mark to them, we're on the edge of our seats wondering: will they or won't they be captured? Also, will they repent of what they've done and try to make amends? Or will they live this forest life forever?
Denouement
Tristan returns Yseut to King Mark.
Well, that answers one of our questions. It turns out that the love potion only lasts for two years and, once it's worn off, Tristan and Yseut are no longer willing to suffer any indignity for love. Tristan returns Yseut to Mark. Although there are some mini-plots of near-discovery and clandestine meetings that occur after this, the lovers' lifetime of separation is pretty much a sure thing at this point.
Conclusion
Tristan and Yseut die.
Since the story was all about love, it ends when the lovers end. Both Tristan and Yseut die because of their dedication to love, and to one another; Tristan because he helped his friend Kaherdin carry on an adulterous affair, then thought that Yseut had abandoned him; Yseut because she simply can't go on without Tristan. Their love is undying though, as evidenced by the two intertwining trees that grow up above their graves.