Chrysanthemum and Marigold Marsh, girl detectives, investigate a break-in at their family's mechanical garden, and discover the culprit is a mechanical cat. After learning that the mechanical cat has only a limited power supply, they must find the cat to return it to its owner. But Dover is a big place, and there are Built Boys, a carnival, and a man called Solomon standing in their way. They'll need help from Inspector Gaspard Greymoor and his constables in order to rescue the mechanical cat before it's too late.