Tristan and Isolde (from Desiree)

There are several versions of the myth. But it is known to have
significantly influenced the Lancelot/Guenevere story.

Here is a lovely summary in the form of a song by Irish singer Heather Dale.
http://www.youtube.com/watch?v=EGif9VMEFaA

And alternatively, here is a written account with a slightly different
telling. (A note: Blanchemaine as she is referred in the song, translates
to "White Hands")
http://www.mythencyclopedia.com/Tr-Wa/Tristan-and-Isolde.html