Billy and Sheena Deep are spending their summer vacation on the Caribbean island of Ilandra with their uncle named Dr. Deep who goes simply by Dr. D. While snorkeling, Billy is nearly attacked by a Sea Monster and manages to get away. Dr. D's assistant, a muscular graduate student named Alexander DuBrow, serves up lunch to the children: chicken salad sandwiches. When Billy is searching for the mermaid, he is attacked by the Sea Monster which he encountered in the beginning of the novel.