At the beginning of Robert Arthur's tenth and final novel in his classic mystery series, Jupiter Jones buys an old steamer trunk at an auction. The trunk originally belonged to a magician who has gone missing and contains a skull named Socrates, which is reputed to talk. But the boys soon discover that others want the trunk, too - including Three-Finger Munger, Maximilian the Mystic, and a mysterious woman named Zelda. Does the trunk hold the clue to the Great Gulliver and the money that has gone missing? What does Socrates have to say about that?