When eccentric Thaddeus McClintock invites Frank and Joe Hardy to accompany him on a sea voyage, the teen-age investigators become entangled in a web of mystery. Who is trying to block the three from securing reservations on freighter ships that carry passengers? Does Mr. McClintock have unknown enemies? Or is there a sinister conspiracy afoot to keep Frank and Joe from going on the trip? Is Captain Harkness's report about sighting a phantom freighter just a figment of his imagination?

The determined efforts of Frank and Joe to find the answers lead to a hazardous game of wits with a ring of slippery smugglers and to a dramatic confrontation on the high seas. At the same time, the boys help their famous detective father solve his current case involving forgers who are selling counterfeit historical documents.

Here is an exciting action-filled mystery that will keep the reader on edge with suspense.