From the author of the much-loved novel Mr Penumbra's 24-Hour Bookstore comes a follow-up about a life-changing loaf of bread. Lois Clary, a software engineer at a San Francisco robotics company, codes all day and collapses at night. When her favourite sandwich shop closes up, the owners leave her with the starter for their mouthwatering sourdough bread. Lois becomes the unlikely hero tasked to care for it, bake with it and keep this needy colony of microorganisms alive. Soon she is baking loaves daily and taking them to the farmer's market, where an exclusive close-knit club runs the show. When Lois discovers another, more secret market, aiming to fuse food and technology, a whole other world opens up. But who are these people, exactly?