Sam and Marcia are back in Edinburgh with Zara, Ben and Professor Ampersand.

The friends are helping to build an amazing electric-powered flying boat when they stumble across an old photograph - a clue that leads them on to the trail of a long-lost secret invention that could change the world.

But a shadowy organisation is also after the invention.

Soon the children are caught up in a perilous struggle that will take them through dimly-lit museum galleries, up into the skies above Europe, down to the murky depths of Lake Geneva, and into the heart of a fortified Swiss castle.

This is a breathtaking adventure of encrypted messages, aerial chases and astonishing secrets from the author of "The 7 Professors Of The Far North".