After defying the ruthless Enclave, surviving the wasteland, and upending the rigid matriarchy of Sylum, Gaia Stone now faces her biggest challenge ever. She must lead the people of Sylum back to the Enclave and persuade the Protectorat to grant them refuge from the wasteland. In Gaia's absence, the Enclave has grown more cruel, more desperate to experiment on mothers from outside the wall, and now the stakes of cooperating or rebelling have never been higher. Is Gaia ready, as a leader, to sacrifice what--or whom--she loves most?