Once again Frank and Joe Hardy take on a puzzling case when their famous detective father asks the boys to assist him in tracking down a notorious jewel thief and his accomplices. The trail leads to the outskirts of the Hardys' home town and to a weirdly guarded mansion on the old Perth estate. With their pal Chet Morton, the brothers must tackle the mystery of the mansion heir's sudden death. A disappearing floor, a huge, savage-looking, hound, a galloping ghost, and a college professor's startling invention are just a few of the strange elements that complicate the boys' efforts to solve both mysteries.