Recession has shuffled Clay Jannon out of his life as a San Francisco Web-design drone and has landed him a new job working the night shift at Mr. Penumbra's 24-Hour Bookstore. There are only a few customers, but they come in repeatedly and never seem to actually buy anything; instead they simply borrow impossibly obscure volumes from strange corners of the store, all according to some elaborate, long-standing arrangement with the gnomic Mr. Penumbra. The store must be a front for something larger, Clay concludes, and soon he's embarked on a complex analysis of the customers' behaviour.