When fifteen-year-old Edna Mather tears an expensive and unfamiliar pocket watch off her little brother's neck, he crumbles into a pile of cogs right before her eyes. Horrified, Edna flees for help, but encounters Ike, a thief who attempts to steal the watch before he realizes what it is: a device to power Coglings-clockwork changelings left in place of stolen children who have been forced to work in factories.
Desperate to rescue her brother, Edna sets off across the kingdom to the hags' swamp, with Ike in tow. There, they learn Coglings are also replacing nobility so the hags can stage a rebellion and rule over humanity. Edna and Ike must stop the revolt, but the populace believes hags are helpful godmothers and healers. No one wants to believe a lowly servant and a thief, especially when Ike has secrets that label them both as traitors.
Together, Edna and Ike must make the kingdom trust them or stop the hags themselves, even if Ike is forced to embrace his dark heritage and Edna must surrender her family.