Two years after surviving an assassination attempt, seventeen-year-old Ella and her team of psychically-gifted agents navigate political turmoil and personal growth while maintaining their cover as wealthy teenagers, until the suicide of a friend leads them to encounter Asherina—a telepathic member of an ancient underground human species. Asherina reveals that her people have been hiding from surface dwellers for half a million years and warns of an impending solar catastrophe, proposing to combine their underground cities with the team's planned tunnels to save humanity from both an approaching war and a solar explosion that could destroy all surface life.