Geoff and Tom are best friends. Geoff is sporty and adventurous, Tom is cautious and a bit of a worrier. They make a most amazing discovery, they find in a cave a flying machine (Aqvila). The cave soon becomes a major archaeological site, but the boys tell no-one about Aqvila which they have found has an invisibility switch. Their later bizarre and hilarious adventures in Aqvila can be unseen.