You're playing Sim City; your sims, however, are bacteria-sized, intelligent individuals—and they're not in your computer, but physically inside your head! That's the premise of Slonczewski's sort-of sequel to her 1998 yarn, The Children Star. Artist Chrys of the city Iridis on planet Valedon, her work blocked and frustrated (she can see infrared, but struggles to communicate her vision) accepts a colony of “micros,” ring-shaped intelligent bacteria from planet Prokaryon. Artists and mathematicians, the colonists call themselves Eleutherians and take up residence in the lining of Chrys's brain. The micros regard Chrys as God, but they're constantly tempted to tweak her brain chemistry to give her extra jolts of pleasure-enhancing dopamine. Daeren, Chrys's mentor, carries a colony of “blue angels”; his other concern centers on rogue micro colonies, who enslave and consume their hosts in pursuit of Endless Light, a condition where the host is continuously blasted with dopamine. Since the micros require arsenic, the slaves often turn into vampires, searching helplessly for their next meal. Worst of all, some slave colonies are clever enough to conceal their presence from everyone save another host—and some of these hosts occupy positions of great power and influence in the government.
Let's face it, individual bacteria can't be smart. But push that difficulty to the side, and Slonczewski's fascinating, flawlessly developed, meticulously detailed scenario makes it easy to suspend disbelief.