Opening Passage: I'll give the full opening passage, despite its length, since it is the key background story. I'll take it from Belloc's original version, rather than the version supplemented by Paul Rosenfield, which is slightly longer.
My lords, if you would hear a high tale of love and of death, here is that of Tristan and Queen Iseult; how to their full joy, but to their sorrow also, they loved each other, and how at last they died of that love together upon one day; she by him and he by her.
Long ago, when Mark was King over Cornwall, Rivalen, King of Lyonesse, heard that Mark’s enemies waged war on him; so he crossed the sea to bring him aid; and so faithfully did he serve him with counsel and sword that Mark gave him his sister Blanchefleur, whom King Rivalen loved most marvellously.
He wedded her in Tintagel Minster, but hardly was she wed when the news came to him that his old enemy Duke Morgan had fallen on Lyonesse and was wasting town and field. Then Rivalen manned his ships in haste, and took Blanchefleur with him to his far land; but she was with child. He landed below his castle of Kanoël and gave the Queen in ward to his Marshal Rohalt, and after that set off to wage his war.
Blanchefleur waited for him continually, but he did not come home, till she learnt upon a day that Duke Morgan had killed him in foul ambush. She did not weep: she made no cry or lamentation, but her limbs failed her and grew weak, and her soul was filled with a strong desire to be rid of the flesh, and though Rohalt tried to soothe her she would not hear. Three days she awaited re-union with her lord, and on the fourth she brought forth a son; and taking him in her arms she said:
“Little son, I have longed a while to see you, and now I see you the fairest thing ever a woman bore. In sadness came I hither, in sadness did I bring forth, and in sadness has your first feast day gone. And as by sadness you came into the world, your name shall be called Tristan; that is the child of sadness.”
Summary: The tale of Tristan and Iseult is perhaps famous enough that there is no need to re-tell it; but it is worth a moment to re-trace its lineaments nonetheless. Tristan, nephew of King Mark of Cornwall, comes to serve his uncle. Hardly has he arrived when he takes up the challenge to fight the greatest champion of Ireland for his uncle's honor and to win the right of Cornwall to be free of tribute; he kills the champion but is wounded with a poisoned sword. (Poison is a theme throughout the story.) Tristan begs his uncle a last favor, which is simply to go out to sea, perhaps to find some land where he can be healed, and it is granted; he is too weak to use sail or oar, so he is simply laid in the boat with his harp and set adrift. Eventually he is discovered, on the verge of death, by fishermen, who bring them to their lady, a generous woman of extraordinary ability in herbs and the like, which she had learned from her mother. The lady's name was Iseult the Fair, or Iseult of the Golden Hair, and she was a princess in Ireland. Not realizing it, she healed the wounds of her country's greatest enemy, the wounds inflicted by her country's greatest champion. He returns home for a while, and, because Mark, having no son, treats Tristan as if he were a son, some of Mark's other barons are jealous. They attempt to get him to marry, but as it happens their attempt leads to Tristan setting sail to retrieve for Mark Iseult the Fair. He does this by slaying a dragon, thus both managing to remain alive once the Irish know who he is, and get Mark a queen, and establish peace between Ireland and Cornwall. His handling of it, however, already shows a devious streak. All this is merely preparatory.
Iseult's mother gives Iseult's lady-in-waiting a special philtre. This potion is to be placed in the cups of Mark and Iseult on their wedding night; and its result will be to make them love each other wholly for their whole lives. Due to mischance, Tristan and Iseult happen to drink of it, and thus the second phase of the tale begins. They are both seized with an obsessive love. Up to this point, Tristan has done everything for honor and his king and uncle. These never cease to be important to him; but now he feels bound to Iseult in his every thought and desire. It is the beginning of the destruction of everything Tristan is; and it is also the beginning of the destruction of Iseult, as well. The philtre, of course, is not a mere appurtenance; it is utterly essential to the course of the tale. What seizes Tristan and Iseult is in itself not their fault, and it is not in itself resistible. This is the primary reason the two lovers can be reasonable objects of sympathy through the rest of the tale; the madness that has seized them will end up degrading them in ways neither could have imagined, but precisely because it is a kind of madness it shields their honor from their shame. In some versions of the tale it is made clear that at some point their actions are motivated less by the madness of the philtre than by their complacency in the deed, so that necessity transforms into an excuse for sin, as it often does; but Bedier carefully leaves it more ambiguous, so that by default compulsion remains the most obvious explanation until the end -- but it is notable that Tristan and Iseult both refuse penance.
The tale continues from there with many twists and turns. There is much trickery and both Tristan and Iseult expend considerable ingenuity in deceiving Mark and others without actually lying. They are not exactly in a state conducive to prudence, however, and they eventually are caught and have to go on the run. This problem is resolved, but they end up in a forced separation, painful to them both. For this is a significant feature of their love: they literally love so much it hurts, "Anguish without end". And yet -- when each is offered a chance to ease the hurt, each gives it up because they each refuse to be comforted if the other suffers.
The third phase of the story sees them both increasingly irrational due to their separation. They both become paranoid that the other might be falling out of love. Tristan, for reasons that are obscure -- but all their reasons become increasingly obscure -- and to a woman who, rather ominously, is named Iseult as well, Iseult of the White Hands. One of the important aspects of the story is the sheer burden Tristan and Iseult place on the two originally innocent people in the arrangement, Mark and Iseult of the White Hands; Iseult the Fair and Tristan marry them already betraying them. And yet, because Mark truly loves Iseult the Fair and Iseult of the White Hands truly loves Tristan, they both exhibit the same symptoms, the same irrationalities, and they both do terrible things. The only thing that makes them less sympathetic is that they were not poisoned by the philtre.
Tristan eventually is wounded by a poisoned spear, and symmetry asserts itself. Tristan sends for Iseult the Fair; but he fears that she no longer loves him, so he arranges a sign. If the ship returns with white sail, Iseult comes; if the ship returns with black sail, she does not. Unfortunately, Iseult of the White Hands overhears the arrangement and its reason, and when the ship appears on the horizon with white sails, she reports that its sails are black. Tristan dies of grief even as Iseult the Fair comes to shore; and she dies of grief beside him. The philtre as a poison was long in acting and there was never any guarantee of when it would achieve its mark; but it was an effective one, and more effective than the poisons that were designed to kill. They at least had a cure.
In passing, talking about some of Mark's barons, who were almost obsessively out to foil the two lovers, the narrator describes how they, too, will die, and says, "Thus God who hates all excess will avenge the lovers on their enemies" (p. 97). It is a remarkable thing to say in a story in which no one acts with moderation. But it is consistently true that the only excesses in the story that do not automatically call forth their own punishments are those committed early on by Tristan and Iseult, operating under the influence of the philtre. And the implacable enmity with which the narrator regards the barons is not entirely misplaced. They are the ones who started it all. Tristan destroyed the Irish champion because the barons would not; their jealous intrigue leads to Tristan's return to Ireland; and when he returns they aggravate every situation. They don't genuinely care about the honor of Mark, as Tristan and Iseult both do even while disgracing it; they are doing it to maintain their power and influence in the kingdom, and to get rid of Tristan, who seems likely to be Mark's heir and yet is too talented to kill directly. It's a twist that I think can easily be lost in the rest of the story: Tristan and Iseult do what is dishonorable but nonetheless struggle to do what is honorable, whereas the barons do what is superficially honorable out of dishonorable motives. And thus we see another aspect of it: despite the wild excesses of Tristan and Iseult -- they are trying to be moderate. We see this reflected in the fact that, to the end, they are both comforters of the poor and suffering around them. It's just that under the circumstances there is no way they can succeed very well. The barons, on the other hand, never try to moderate themselves; they only stay their hand out of fear or calculation.
Perhaps the most interesting thing about the story is how it reflects the paradox of love. Love raised to passionate excess is folly, shame, death. Yet something in love seems only to attain perfection when it is love so great one may die of it. The tale itself provides no solution, only the paradox. Well, perhaps it does suggest a solution, in Ogrin the Hermit's insistence on penance; but of that route the story can tell us nothing.
One day when the wind had fallen and the sails hung slack Tristan dropped anchor by an Island and the hundred knights of Cornwall and the sailors, weary of the sea, landed all. Iseult alone remained aboard and a little serving maid, when Tristan came near the Queen to calm her sorrow. The sun was hot above them and they were athirst and, as they called, the little maid looked about for drink for them and found that pitcher which the mother of Iseult had given into Brangien’s keeping. And when she came on it, the child cried, “I have found you wine!” Now she had found not wine — but Passion and Joy most sharp, and Anguish without end, and Death.
The Queen drank deep of that draught and gave it to Tristan and he drank also long and emptied it all.
Brangien came in upon them; she saw them gazing at each other in silence as though ravished and apart; she saw before them the pitcher standing there; she snatched it up and cast it into the shuddering sea and cried aloud: “Cursed be the day I was born and cursed the day that first I trod this deck. Iseult, my friend, and Tristan, you, you have drunk death together.”
Recommendation: The tale in general is one of those that every educated person should have read in some form, and Bédier's is easily the most accessible and coherent of the versions. Highly recommended.