Biggles and the Black Mask -
Captain W.E. Johns
1964 - Hodder and Stoughton, London - First Edition
Biggles follows a hunch to check up on a dark character from this past, and in the process uncovers a money smuggling ring run by a bored doctor.
The smuggling ring is being run under the guise of a charter holiday service. Ginger is sent along as a customer to check it out, and spends a few days in Nice before being kidnapped when he gets a bit too close to the truth.
The rest of the team manage to uncover Ginger's location, crack the smuggling ring and spring Ginger from his imprisonment in a house in Essex. The leader of the gang, Doctor Fortescue, confesses to running the operation purely as adventure, and then commits suicide.