During the early part of the Cold War there was a great rivalry between factions over areas of control. Biggles and the team are asked to look over the islands of the Southern Indian Ocean and find Russian interest evident. They pick up a castaway and end up battling a Russian submarine and its crew, with the help of Marcel Brissac.

goodreads summary

Biggles and the team are sent to investigate the possibility that agents from behind the Iron Curtain are being placed on isolated islands in the world's great Oceans. These islands are still the territories of various countries such as the United Kingdom and France and, although uninhabited, will have great strategic importance as refuelling stations for aircraft as world travel increases.