When we are introduced to Matt the Cabin Boy, he is rescuing an old man from his failing hot air balloon in a daring leap 800 feet over the ocean. Something tells us this won't be a story about how he learns to overcome a fear of heights.
Fast-forward a few months and they're embarking on a routine trans-Pacificus journey when a young, wealthy passenger named Kate de Vries (and her shrill chaperone Miss Simpkins) make a daring effort to board the airship Aurora via ornithopter. Matt immediately recognizes a kindred spirit and they become friends, especially after they realize that the man Matt rescued was Kate's grandfather, who had potentially discovered a brand-new species. Kate is there in the hopes that they can verify her grandfather's discovery… but before they get a chance, the ship is attacked by a crew of pirates, led by none other than the dread Captain Szpirglas.
The pirates try to leave after a successful pillaging, but a strong gust of wind forces their ship into the Aurora, tearing into her hull and badly damaging the gas cells that keep her afloat. The ship has no choice but to crash land on an uncharted island—which coincidentally (or, you know, conveniently) happens to be the same island over which Kate's grandfather spied that mysterious new creature.
During their time on land, Kate and Matt and junior sailmaker Bruce Lunardi make several expeditions into the forest—where they discover not only a complete skeleton of the strange puma/bat hybrid they name cloud cat, but also a living cloud cat specimen that Kate wants to track down. In the process they discover an untapped source of the precious hydrium gas that they need to refill the Aurora's damaged gas cells, but also that they have crash-landed on the same island where the pirates live in their secret home-base. Oh goodie.
The pirates take Matt and Kate hostage shortly after Bruce is attacked by a cloud cat. (In the creature's defense, it was confused—Bruce had been handling dead fish and, as such, reeked of the cloud kitty's primary food source). Matt and Kate are thrown into a hydrium pit to die, but make a daring escape… only to find that the pirates have taken the Aurora yet again, but this time they are playing for keeps.
Matt, Kate, and a wounded Bruce concoct a daring plan to take back the ship involving a sleeping potion, a fish soup, and lots of luck. Bruce is killed by the pirates in the course of the action (bummer), but Matt and Kate manage to get the ship airborn, kill/injure/capture the rest of the pirate crew, and Szpirglas meets his fate after a tussle with Matt on the top of the airship's back.
All of this adds up to a happy ending for our two protagonists (R.I.P. Bruce). Matt is finally able to attend the prestigious Air Academy without worrying about how he will provide for his mother and sisters back home, and Kate wins scientific acclaim when she presents her findings of the Cloud Cat.