Penny Parker goes on vacation with her father and quickly becomes engrossed in a search for foreign spies! What are the spies up to, and what information are they transmitting over the radio? And, how does her father's new friend fit into the whole picture? Follow Penny as she solves the mystery of The Voice from the Cave!This book is from the Penny Parker series by Mildred A. Wirt, also known as ghostwriter Carolyn Keene, of the Nancy Drew series of books.