Doug and Becca MacKenzie, two teenage orphans, are forced to live with their uncle after their parents mysteriously disappear. Their uncle, Fitzroy MacKenzie, is the sea captain of a research ship named Expedient. Doug and Becca over hear Fitzroy talk to an unusual passenger, and decide to do some snooping about. As they learn, Expedient is not a research ship at all. It is a World War II Q ship. As a Q ship, the Expedient is disguised as a defenseless cargo ship, but once an enemy ship comes within firing range, cleverly hidden guns rise above the deck and open fire. Doug and Becca are caught, but Fitzroy offers no explanation for the guns. Soon after, he effectively removes Doug and Becca from his ship. Doug and Becca, back on dry land, are determined to find out if their parents are still alive. They follow a mystifying set of clues that leads them on an extraordinary journey.