Biggles opens the story looking carefully at forged stamps and banknotes. He is quickly faced with an attempted bribe and worse. Bertie and Ginger trace the wrongdoers to Africa before Tug Carrington and then Biggles arrive on the scene.

goodreads summary

A Story Of Sergeant Bigglesworth CID And His Special Air Police: The scene would not by ordinary standards have been judged remarkable. It was remarkable only in that Ginger, in all the years that he had known his chief, had never seen him reveal more interest in a postage stamp than was required to stick this on an envelope.