As Tristan lies dying in his bed, he asks his wife to look out the window and tell him whether the sails of an approaching ship are black or white. If the sails are white, he knows it's his messenger's way of telling him that he returns with Tristan's lover, Yseut, who he believes is the only person capable of healing him of his poisoned wound.
But Tristan's wife (oh dear) knows of her husband's secret arrangement and, out of jealousy, tells him that the sail is black when it is actually white. Believing that his lover has failed him in the end, Tristan dies before they can be reunited. Later, Yseut dies of grief in his lifeless arms.
We know, we know. You've seen that before. But this is where it all came from.
And it's one of the most tragic endings in literature, mainly because of its dramatic irony. Dramatic irony is a fancy way of describing a moment in a story or play when the audience knows something that the character doesn't. We know the approaching sails are white, because Béroul tells us so. If only we had some way of telling Tristan that! Oh, the dramatic irony of it all!
One of the most interesting minor characters in the Tristan legend plays an important role in this ending. That would be Tristan's wife, the "other" Yseut (of the White Hands), who lies about the color of the approaching sails out of jealousy. (Okay, but seriously, how many Yseuts are there in the world at one time? We haven't met any.)
Tristan has refused to consummate his marriage with Yseut of the White Hands because of his devotion to his first love. But, hey, that's not all! He has even convinced his wife's brother that Yseut the Fair is a worthier love interest than his sister. Ouch.
At the very end of the story, Yseut of the White Hands finally lashes out against her fate. She's been pretty silent until now, but when she makes her presence known she does it in a big way. The enormity of her impact on the ending makes us suddenly aware that Yseut of the White Hands is a person, too—one with normal human feelings.
Just when we should feel most sorry for the two main characters, we realize the negative consequences of their ill-fated romance on Yseut of the White Hands, whose life it's basically destroyed. Her presence in the ending forces us to reflect on what's destructive about Tristan and Yseut's love even as we're encouraged to contemplate what's beautiful and noble about it.