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 living in a mobile home on the beach.
One morning Benny finds a valuable gold locket in the sand and decides to try to find its owner. Soon, the Boxcar Children are caught up in a puzzle that involves shadowy figures on the beach at midnight and the mysterious Miss Smith who seems to live alone in the big Tower House nearby....