Mabel and Will Greystone are looking forward to an exciting holiday with their aunt Lucy, a famous relic hunter in the land of Marvolia. Using an ancient map, they are searching for the legendary lost town of Umber, famous for the invention of clockwork, which was mysteriously abandoned hundreds of years ago. The only other clue they have is a strange old key left behind by the last inhabitant of the town, but they have no idea what the key is for. Also on the hunt for Umber is devious Byron Cogsworth, desperate for the glory such an important discovery will bring. Can Mabel, Will and Aunt Lucy find Umber before him? And what secrets will the clockwork key unlock?