Sam is back in Edinburgh with his brilliant uncle, inventor Professor Ampersand, and cousins Zara, Ben and friend Marcia. Their efforts to build an experimental flying boat leads to a dramatic discovery - a long-lost secret invention that holds the key to anti-gravity. But a ruthless and sinister organisation will do anything to own the machine and Sam and his friends must stop them at all costs . . .

So begins an adventure of nocturnal museum thefts, encrypted clues, murder cover-ups, mysterious airships and sky-chases over the rooftops of Edinburgh.