Lower Rhumbsford is a city far removed from its glory days. On the banks of the great river Rhumb, its founding fathers manipulated the river's mighty flow in feats of hydraulic prowess that would come to power an empire. But a thousand years have passed since then, and something is wrong. The once torrential Rhumb has been reduced to a sluggish trickle. The fortunes of the Locke family are similarly reduced. In a once fashionable quarter of the once great city, siblings Samuel and Briony Locke distract themselves: Sam tends to his vast lock collection instead of finishing his engineering thesis; facing the prospect of a disagreeable marriage, Briony occupies herself with alchemical experiments. Until one night Samuel leaves the house carrying five of his most precious locks and doesn't come back. If brother and sister are to be reunited, they will need the help of a tight-lipped house spirit, a convict gang, an association of antiques enthusiasts, a tribe of troglodytes, the Ladies Whist Club, the Deep State, and a lovesick mouse.