 Geoff and Tom are best friends but very different. Geoff is sporty and adventurous. Tom is cautious and a bit of a worrier. So when they make the most amazing discovery, they deal with it in very different ways. The boys are on holiday in Wales when they find - hidden beneath the body of a Roman centurion in a cave - a flying machine! The cave soon becomes a major archaeological site, but the boys tell no-one about the machine. Back home, the boys keep the machine, Aquila, in the garage, telling their parents it is an old prop left over from a film set. Aquila has a marvellous invisibility switch. Not only can they have all kinds of bizarre and hilarious adventures in Aquila, they can have them unseen.