The Legend

King Mark of Cornwall sends his nephew Tristram to Ireland to escort his (Mark's) intended bride Issolde to Cornwall. On the ship on the return journey Tristram and Issolde fall in love under the influence of a potion prepared by Issolde's mother for her wedding night. King Mark and Issolde are married as planned but the passion of Tristram and Issolde for each other cannot be long concealed. 

Tristram flees from the jealous Mark. He then meets Issolde 2, who falls in love with him. They marry, but Tristram's love for Issolde 1 runs deep and the marriage remains unconsummated. Tristram is wounded by a poisoned arrow.  He can only be saved by his true love, Issolde 1. Tristram sends for her, begging her to come and quickly. He tells her to show a white sail if she is coming.  If the ship has a black sail he will know there is no hope. she sets sail immediately but Issolde 2 falsely tells Tristram that the sail is black.

Tristram was a real sixth-century British king who lived at Castle Dore in Cornwall. King Mark was his father, not his uncle. He lived three generations after Arthur.