Biggles protects a secret message!Following a bad bout of fever, Biggles is taking it easy. His doctor has advised him that a sea voyage will do him good, and so he is relaxing aboard the S.S. Stavritos bound for Athens, with Algy and Ginger for company.

It is not all smooth sailing, however. Disaster occurs when a fighter plane attacks and crashes into the deck, sinking the ship and forcing everyone on board to swim to safety.

Exhausted but alive, Biggles and the gang make it to the shore of Barcelona but they have swapped the frying pan for the fire. A chance encounter with a British intelligence agent, a hunchbacked assassin and a mysterious coded document lands them in a web of espionage.

With enemies at every turn, can Biggles, Algy and Ginger make it back to England alive, and keep the document from falling into the wrong hands?

Amidst the Spanish Civil War, Biggles must face his most dangerous mission yet!