A fourteen-year-old boy, John Jacob Turnstile, has got into trouble with the police on one too many occasions and is on his way to prison when an offer is put to him - a ship has been refitted over the last few months and is about to set sail with an important mission, The boy who was expected to serve as the captain's personal valet has been injured and a replacement must be found immediately.

The deal is struck and Turnstile finds himself on board, meeting the captain, just as the ship sets sail.