From the back of the book:
Henry, Jessie, Violet, and Benny used to live alone in a boxcar. Now they have a home with their grandfather and are spending the summer in a lighthouse on the New England coast.
Jessie sees a mysterious woman walking on the grounds of the lighthouse late at night. Watch, Jessie's dog, wakes up growling almost every evening-at midnight. And Henry finds a puzzling note in the sand....
Many more strange things will happen before the Boxcar Children can solve The Lighthouse Mystery.