Isolde's day has come. In Ireland, her mother, the Queen, lies dying. The throne of the Emerald Isle, one of the last strongholds of the goddess, awaits her. But while Ireland is her destiny, Isolde is already Queen of Cornwall, trapped in a loveless marriage to its mean-spirited King Mark. Her true love is his nephew, Tristan of Lyonesse, who has never married, remaining faithful only to Isolde.

Across the sea in France, a young princess who shares Isolde's name enters the story. King Hoel named his daughtor in honor of Isolde of Ireland, and young Isolde of France has always been determined to outdo her beautiful namesake. She is a physician, too, and is called "Blanche Mains," for her white hands and healing touch. Blanche is of an age to be married, and she has chosen her husband—Tristan of Lyonesse. Her father objects, but fate favors Blanche. King Mark has become suspicious of his wife and nephew, and when Tristan is wounded in battle, he sees a chance to separate them for good.

Mark sends Tristan to France to be healed by Blanche, who makes the most of the opportunity. Tristan's letters to Isolde are intercepted, and he is told that she has given him up. Near death from his wounds, Tristan sends one last, desparate letter to Isolde by a trusted servant. He is dying, he tells her, and asks for one final sign of their love. If she can forgive him for betraying her, she must come to France in a ship set with white sails. If the ship's sails are black, however, he will know that she no longer loves him. Isolde immediately leaves for France, but when Blanche sees the white-sailed ship from the castle window, she pulls the curtains and tells Tristan that the sails are black. To her horror, he turns his face to the wall and dies.

There ends the traditional medieval story of Tristan and Isolde—with betrayal, death, and grief. But the original Irish lengend ends differently, and so does this book, wth magic and drama as only Rosalind Miles could write it.