The Aldens are visiting Aunt Jane in Elmford. She tells the children that she has tickets for them to see a mystery play at the Trap-Door Theater. A theater patron who loved mysteries left her life savings to improve the theater and to hold a yearly contest with a cash prize awarded to the best mystery playwright in town. The Aldens can't wait to find out everything there is to know about the old Trap-Door Theater. But there is more than one mystery going on. Each day at rehearsal something strange has happened to the front-row seat once occupied by the benefactor—popcorn all over the floor, an old costume laid out, a spotlight shining on her empty seat. Is the benefactor's ghost haunting the theater, or is one of Elmford's living residents up to no good? There is only one thing the Aldens like better than a mystery—two mysteries for the price of one!