Three years after she helped overthrow the Catchkeep-priest, Isabel settles in the small town of Sweetwater. But when a hostile army approaches, Isabel is tasked with protecting the children of Sweetwater in an underground tunnel maze built as a centuries-old experimental program called the Latchkey Project. With the help of some old friends, including the ghost of a supersoldier from the Before-time, Isabel has a new mission--to free the ghosts from the Before-time who are trapped in the maze's purgatory.