Where Are the Children? by Mary Higgins Clark

To escape the terrible secret in her past and the people who knew about it, Nancy Harmon changed her hair color and style, moved across the country to Cape Cod and married Ray Eldredge, believing that at last she had begun a new life. Seven years have passed and she is happy with her home, her husband and her two beautiful young children, Michael and Missy. Then one day an article appears about a famous California murder trial along with a picture of a woman who looks suspiciously like Nancy. That very mourning her two children mysteriously disappear from the yard, and past and present become inexorably linked.