Biggles and the team set out as part of the International Squadron to help the Finns. They stumble upon a dying Pole and become involved in a tussle with German intelligence for his papers on metal alloys for aircraft.

goodreads summary

The Air Ministry have allowed Biggles, Algy, Ginger and Smyth to accompany a party of volunteers to help Finland in their struggle against Soviet aggression. Whilst on a reconnaissance flight, Biggles finds a dying Polish scientist called Petolski, who had been working on important experiments with metal alloys for aircraft. He had the results of seven years' labour in a portfolio with him and rather than let the research fall into German hands, (Poland having been invaded by Germany), he has hidden it.