The Great Recession has shuffled Clay Jannon out of his life as a San Francisco web-design drone, and serendipity, sheer curiosity and the ability to climb a ladder like a monkey have landed him a new gig working the night shift at Mr. Penumbra's 24-Hour Bookstore. But after just a few days on the job, Clay begins to realize that this store is even more curious than the name suggests. There are only a few customers, but they come in repeatedly and never seem to actually buy anything, instead ' checking out' impossibly obscure volumes from strange corners of the store, all according to some elaborate, long-standing arrangement with gnomic Mr. Penumbra. The store must be a front for something larger, Clay concludes, and soon he has embarked on a complex analysis of the customers' behaviour and roped his friends into helping him figure out just what's going on. But once they bring their findings to Mr.Penumbra, they discover the secrets extend far beyond the walls of the bookstore.