I wasn't surprised to see that this Biggles title was originally published under the title of 'The Rescue Flight' because, somewhat unusually, Biggles plays a secondary role in the adventure.
Two school pals realise that they are missing out on the war and when one of them finds out that his brother is captured behind enemy lines, the pair decide to run away and joine the Royal Flying Corps.
They use uniforms of the brother and borrow two 'planes to get to an RFC unit. There they meet up with Biggles and are eventually obliged to tell him the truth.
From then on he shields them and assists them in the rescue
mission(s). It gets very fraught and develops into something more than a simple rescue. There are thrills and spills with plenty of dog-fights along the way before 'Thirty', so called because he is the younger brother of Fortymore, nicknamed 'Forty', the captured pilot, finally, with the help of Biggles of course, completes the rescue flight.
However, the fact that he is still technically a school boy is then discovered by the Commanding Officer so there is that little problem to be sorted out before a happy ending!