Sub-Lt. Peter Clayton joins his new ship, the cruiser, H.M.S. Canterbury, and is delighted to find thereon his 2 friends, Midshipman Billy Middleton and Sub-Lieut. Jimmy Birchett. The ship is patrolling thr Mediterranean, hoping for a glimpse of the Italian Navy. A chance landing on one of the many islands brings Peter into contact with the famous secret agent, Captain Arkwell. Thereafter adventures follow in quick succession.