During Mr. Hardy's investigation of a ruthless gang of terrorists, Frank and Joe witness an explosion in the sky near an airborne dirigible owned by Quinn Air Fleet. They decide to look into the matter. The first clue takes them into a new animal park outside Bayport, where they are lured into a trap by an unknown enemy. Pop Carter, the park's owner, has problems, too. He is being pressured into selling out by a competitor as well as a real-estate firm that wants the land for other purposes. At the same time, strange occurrences frighten both visitors and animals. Frank and Joe are warned to stay out of the case, but follow up another clue. It leads them to an abandoned island, where they barely escape severe injury. When they return, they find a threatening message on their front door, written in an Oriental language. Do they have to contend with yet another adversary? They face danger from all sides, but their superior sleuthing skills and their courage win out in the end, when they unravel the threats of this many-faceted mystery. - Flyleaf.