On the scene following an explosion which destroyed a non-American submarine in the Gulf of Mexico off Sarasota, Bill Barret, about to enter the Naval Academy, retrieves a drifting bottle which gives a clue to the whereabouts of navy man Richard Callahan, who had disappeared. The bottle was to lead Bill on the trail of the strange vessel which had been sighted recurrently and then disappeared. With Commander Stevens, Bill goes to St. Thomas, in the Virgin Islands, to establish the nature of the strange craft -- a key to enemy offensive work. This is a formula story reminiscent of the old Jack Armstrong adventures of radio. The author's name adds an authoritative note.