When twelve-year-old John Barr is forced to join the navy his life takes a dangerous turn. As he trains to become a powder monkey on board the mighty HMS Fearless, he soon learns that the ship hides many secrets.

Thrust into a sinister world of spies and covert operations, John must go ashore accompanied by fellow shipmate Kit. But when their mission goes awry, the friends are abandoned and watch from the shore as the Fearless sail away. Can they now survive behind enemy lines?