This story introduces a mystery on almost the first page, when the Hollisters attend an auction sale at Stone Farm near Shoreham. When Mr. Stone died he had only one relative, his grandson, Graham. Graham had been informed that he was to have the one thing of value on the farm, known only as "mystery boy." Unfortunately no one knew who or what "mystery boy" was. The Hollisters, therefore, were quite surprised to find that the hobbyhorse they purchased at the sale held the key to the identity of this strange bequest. This, however, is a minor mystery, for when the Hollisters visit Pony Hill Farm, a horse farm owned by cousins of Mrs. Hollister, Ruth and Charles Thomas, they all become concerned with a beautiful appaloosa pony that appears at the farm as if out of nowhere. The solution of this puzzle and that of the elusive Graham Stone, along with learning trick riding and entering the County Fair Horse Show, makes a new Hollister story of interesting and unusual adventure.