The countryside has been struck by a string of robberies by an extremely clever and hard to catch pirate. That pirate just happens to moor his boat in a secret creek on Dona's property. Of course Dona stumbles upon it which leads to the meat of the story as the relationship between her and the pirate unfolds.
There are things I don't really like in the book. du Maurier, for all she does to build a strong female lead character, sometimes tosses in sentences that make women seem like domestic weaklings. And, it's sad to see a woman so caught up in herself that she neglects her children. That is just a cold hard fact of life, though.
The writing, though, is absolutely brilliant. She catches the essence of human emotion, of love, of restlessness, and self-discovery. Take, for example, this passage just after Dona realizes what she has with her pirate and how it differs from her love for her husband:
"...and she thought with pity of all the men and women who were reluctant, who were shy, who imagined that passion and tenderness were two things separate from one another, and not the one, gloriously intermingled, so that to be fierce was also to be gentle, so that silence was a speaking without words. For love, as she knew it now, was something without shame and without reserve, the possession of two people who had no barrier between them, and no pride; whatever happened to him would happen to her too, all feeling, all movement, all sensation of body and of mind."
This isn't my favorite du Maurier book. I would recommend Rebecca and My Cousin Rachel before it. But, I do love du Maurier's writing style so much that even a book with an unusual plot is a welcome read. And to write something like this in 1941 shows some guts, as well. Women weren't supposed to blossom outside of their matronly roles.