Biggles and the team travel to Canada to retrieve stolen jewellery after one of the robbers turns Queen's Evidence. From the back of the Knight edition:

Serious trouble brings Dusty Miller to see Biggles, his old squadron leader, late one night. Miller's son, Tommy, once wrongfully spent three months in prison and his bitterness made him potential material for a life of crime. he fell into the hands of Raulstein, a big time crook, and was forced to take part in a big jewel robbery. How can Biggles help, especially when he knows he ought to report Tommy's involvement.

So how can Biggles help? Well obviously by reporting the chap to the Police (that decision took less than 3 pages) and then taking him to Canada to try and retrieve the illgotten gains. This they manage to do, although not before a few run ins with Raulstein and his new gang.