King Henry I of England has decided to end an old feud by marrying Lord Dain of Penruan to the daughter of Penruan's enemy, Gavin of Wroxley. Thinking his bride-by-proxy will be a mere child, Dain is startled to meet the lovely Emma. He's rude to her and refuses to sleep with her. Emma decides to make Dain care for her and sets out to make life easier for the folk of Penruan, who are forced by Dain's religious fanatic mother to live without kindness or help for their ailments.
In the local village of Trevanan, the healer Agatha proves to be an unexpected ally as she recognizes Emma's ability to work magic. In a cave hidden below the cliff where Penruan is built, a stranger who calls himself Hermit offers friendship. On the high cliffs a ghostly woman in white watches Emma. Meanwhile, Dain's mother, Lady Richenda does all she can to keep the two apart.
Emma begins to find small gifts on her pillow almost every morning and believes they are peace offerings from Dain. Gradually, Dain softens toward his unwanted bride, until they make love.
But Lady Richenda has other plans and causes as much trouble as she can, blaming Emma for anything that goes wrong at Penruan. Finally, Dain has had enough. So has Agatha, who is disgusted by Richenda. Agatha reveals the truth about the woman in white, whose name is Vivienne. Emma, also wanting truth, tells Dain about her magical ability.
After Dain decides to send his mother to a convent to live, Agatha, Emma, and Vivienne lay a spell on Lady Richenda, binding her to kindness toward all people for the rest of her life. Dain watches this magical working and understands that all magic is not evil.
When Emma's parents arrive with an army, in response to an urgent letter from Richenda, it's clear she's been working her own kind of evil. Explanations follow and though Dain remains skeptical, at last the feud is over, so he and Emma can live together in peace.