The last book in the series is a bit bland compared with the others. The further into the series, the more bland it got. Spoilers ahead.
Rather a stretch, but Martin and his pals decided to go to the US because they got into the wrong ship in book 4. Once in Texas (where Tommy is from), they witness a barnstormer get killed doing a trick because his planes were in bad shape and he wasn't experienced enough.
They then travel down to Camarillo where they experience a raid by Mexican bandits. Meanwhile Charley is in Mexico because her father had gotten a job there. She decides to come up to visit Martin. However she and a companion are captured by a rebel (or bandit?) who demands 400 rifles and some cash to release her.
While the US government and the British consul are dithering on what to do, Martin and his pals (Tommy, Slingsby and the Russian pilot named Puddy) drive down to save her but one of their cars broke down. They decide to buy the barnstormer's planes from his widow and stage the rescue by air.
Overall it's not too bad as it opens a small window into Mexican revolutionary politics at the time (thank goodness Hennessy did a better job than with the Russian book) and Martin even gets to meet Pancho Villa who captures him but releases him unharmed. It's amazing that the timeline was only about 100 years ago and it was so different then. But as I said in book 1, this series is somewhat like a young adult book because the writing is fairly simplistic.