In an alternate Regency, Kim is scouting out a wagon where an itinerant magician is performing his tricks. Once she knows how long he is out, she goes inside to check what he's got, a spying mission a real toff sent her on. When she tries to open one chest, a ward explodes, and she thinks she should have asked for more money, since she hit on a real wizard.
When she rouses again, the wizard and his servant have caught her. Whereupon the wizard takes her on, over the servant's objection (there are hints later that he's trying to atone, but nothing much comes of that), and sends her back to report on what she found. And he takes her out of London on a mission to the countryside, involving a search for something stolen. She works out the details as the rest of the story progresses through events that include a mad race of carriages, a silly order of would-be druids, a meeting that doesn't meet, a request that someone not ask that a wizard turn him to a frog because that's a lot of work, repeated attempts to burgle a library in one night, an elopement, a priest hole, and the illegitimate half-brother of a nobleman.