Penny Parker takes on a job at the newspaper during her school vacation. But all does not go well as she covers a story of the explosion at the Conway Steel Plant. What caused the explosion? There are plenty of mysteries to be had Follow Penny as she solves the mystery of the Signal in the Dark 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.