Whilst ostensibly on a mediterranean cruise to help Biggles convalesce from a bout of fever the team are thrown, almost literally, into the Spanish Civil War. When their cruise ship is sunk under them, they swim ashore and bump straight into a spy trying to get a message back to England.

Just managing to stay one step ahead of the secret police, Biggles and the rest manage to get the message safely to its destination. But that is not before being imprisoned (twice), Ginger joining the resistance airforce for a few days, and them all spending more time flying the enemy's planes than those of their own side!