After the supposed death of the Dark Queen and the capturing of the Underworld, the Sentinels believe they've won the war. When a strange ally of the Dark Queen named Doctor Zero emerges, he announces that he's discovered how to turn everyone soulless. With him, he brings a fearless army, a dangerous serum, and the powerful Dark Queen by his side. To have a fighting chance against him, Riley Stark frees King Bane and the Renegades from prison. The Dark Queen's old husband-King Lucius, who resurfaces with a bloodthirsty army of his own-disagrees with Doctor Zero's plan to turn the world soulless and seeks revenge. When Caleb is injured in the battle, Riley is forced to trust strangers and neglect her own people to save him. As the government crumbles, new monsters come out of hiding, and dangerous groups take to the streets, Riley must protect Caleb, prevent her people from becoming soulless, and ask herself if she can continue to serve the Sentinels when they've hidden so much from her.