Two boys, Geoff and Tom, are on a weekend away with their mums where they do some digging for treasure at a local moor. As Geoff is digging, he falls into an underground cavern followed cautiously by Tom. It is there they find the skeleton of a Roman Centurion, standing beside a large red object which looks like a giant boulder. One of the boys notices a hollowed out area in this "boulder" which turns out to be a two-seat cockpit. The craft, which is called Aquila, soon turns out to be something more advanced than Roman technology, and by pressing the numerous coloured buttons in the cockpit the boys learn more about this strange craft and take off upwards into the sky.
The story becomes more tense and yet humorous as the boys try to think of ways to hide their amazing discovery, prevent doing damage with it, and communicating with it. Eventually, the boys manage to find a way of communicating with Aquila, but even then the show managed to end each week with a dramatic cliff-hanger as a new problem arose.
The last ever line in the series came as the characters discover the source of Aquila, and the camera pans into outer space to see a massive abandoned spaceship orbiting the sun. They realise the significance of this and exclaim "A battle cruiser! You could have some serious fun with a battle cruiser!".