While on holiday, Tom and Geoff fall into a cave, where they find the skeleton of a Roman Centurion. Alongside the skeleton is a strange pod, which the boys quickly discover is a remarkable flying machine.
With the end of their holiday approaching, Tom and Geoff need to find a way to get Aquila back home without anyone noticing. They come up with a plan but it involves Tom flying Aquila on his own, which terrifies him.
Frustrated at not being able to fly anywhere in Aquila for fear of being seen, Tom and Geoff start to experiment with the buttons on the dashboard, and discover one that makes Aquila and its occupants invisible.
When Tom's hand gets stuck in Aquila's glove compartment, his whole arm turns green. The green skin comes with immunity to pain and incredible strength, which comes in handy when he runs into the school bully.
When warning lights start flashing on Aquila, the boys realize that it is running out of power, but they have no idea what sort of fuel it uses.
Tom accidentally zaps Mrs Murray with a laser beam that freezes her, and the boys need to teach themselves Latin in order to ask Aquila to undo it. Mr Urquart finds an exercise book, in which Tom's written down everything about Aquila.
Mrs Murray complains to the police about the "aliens living next door" and sends photos of Tom and Geoff in Aquila to the local newspaper.