Mosca Mye is an orphan, living with her aunt and uncle in a small town called Chough. Chough "could be found by straying as far as possible from anywhere comfortable or significant, and following the smell of damp... more a tumble than a town." Eager to see the wider world, she sets free a confidence man named Eponymous Clent and follows him to the big city of Mandelion.
The wider world turns out to be a sort of pixilated version of 18th-century England. It's a place where calendar days are sacred to Goodman Palpitattle, He Who Keeps Flies out of Jams and Butterchurns, or Cramflick, She Who Keeps the Vegetables of the Garden Crisp; where the mad duke of Mandelion has fallen in love with the Twin Queens, who were born hand in hand, joined at an extra finger; where coffeehouses sail up and down the river (a fact which later occasions the wonderfully daft command "Stop that coffeehouse!"). A place where people are given names like Eponymous.
Mosca is unprepared for Mandelion, which is riven by plots and factions. The king of the realm was beheaded many years before and the country has fragmented into city-states, each supporting a different monarch in exile. In the meantime the guilds have taken over; the Stationers are the most powerful, with the authority to decide which books are safe to read, and as Fly by Night opens they are so strong that most people are thrown into terror at the mere sight of print. But in recent years they have been challenged by the Locksmiths, a guild responsible for keeping criminals off the streets except for those criminals who work for the Locksmiths themselves. Meanwhile the duke, to show his love for the Twin Queens, is trying to make the city completely symmetrical, and his sister, Lady Tamarind, has her own plans.
Mosca, though, is resourceful and clever, and knows enough about lying to match wits with the scheming Clent. But she soon discovers that she must do more than just survive; the existence of the city is at stake. As she threads her way through the various guilds and factions she hears rumors about the return of the Birdcatchers, a fanatical sect that once sought to suppress the calendar gods and force everyone to worship the Greater Truth. And she learns more about her father's ideas, and why he was forced to leave Mandelion for Chough.
But the plot, wonderful as it is, almost doesn't matter. Mosca flees with Clent because of the way he talks "She had subsisted on workaday terms, snub and flavorless as potatoes. Clent had brought phrases as vivid and strange as spices, and he smiled as he spoke, as if tasting them." and Hardinge, too, has the power to charm with language. (I have to admit I stole the word "pixilated," above, from her.) Every paragraph, sometimes every sentence, is a delight, so much so I'm tempted to quote passage after passage:
There was no escaping the sound of water. It had many voices. The clearest sounded like someone shaking glass beads in a sieve. The waterfall spray beat the leaves with a noise like paper children applauding. From the ravines rose a sound like the chuckle of granite-throated goblins.
"And you know what else I want for my trouble? ... I want your uncle's heart spiked on a boathook so I can hear it crackle as it bakes in the sun."
Or ... okay, okay, I'll stop here, and let you have the pleasure of discovering this book by yourself. Fly by Night reminded me by turns of Joan Aiken, of Lemony Snicket, of James Thurber's marvelous The Thirteen Clocks, of Philip Pullman. In the end, though, it is unique, triumphantly itself.