The first day of summer in Windle Bay is very, very boring, until Beno's homemade catapult sends a red ball straight through the back window of Nell's Hardware Shop. Behind a blue door marked KNOCK (THEN DUCK), Miles and Beno meet Theo, a young inventor with a workshop full of things that almost work, and Bolt, a junk robot who is sure he is a dog. Theo has drawn a digging machine with nowhere to dig. Then wildlife ranger Hazel's tagged marmot goes quiet after a storm, and Old Gus unrolls a fifty-year-old map of a crystal cave inside the mountain. With a Mechanical Mole that cannot reverse, a pocketful of Glow Marbles and an Echo Clicker, the three friends head underground to bring a marmot family home.
