An artist's brain rules a team of a million microbial minds-from the Campbell Award winning author of A Door into Ocean.

In the hundred-level city of Iridis, human lords recycle diamonds and emeralds down to the Underworld while sentient machines pilot lightcraft and perform surgeries. Microbial minds expand the brains of scientists and artists. The artist Chrys directs her microbes to paint startling installations, design smart buildings and fix quantum networks. A world-size virtual intelligence called Transit commissions Chrys to create a whole new city on another planet.

But unexpected flaws fracture her designs. Microbial criminal gangs invade Chrys's brain to kill off her own micros and take control. In Iridis, the roots of buildings grow cancers that escape the Underworld and cause earthquakes. Outlawed machines shake down shops and kidnap human lords for ransom. The city's rulers ignore the signs of collapse, then face a new threat that no mind foresaw.