In 2046, humanity is moving away from using robots and trusting them less and less. During this period, life becomes very perilous for beings known as Seconds. As far as the public knows, Seconds are the most recent line of robots. This is the world in which Detective Ross Sylibus lives. Detective Sylibus is transferred to Mars at his own request after his partner was killed by a robot. Upon arrival, he is immediately drawn into a murder case involving a country singer on his flight. He steps off the plane, observes the unfolding scene, and meets his new partner, Armitage, a female cop with a major attitude. A rash of murders begins when more women are killed. As the investigation continues, a secret is uncovered: there is another line of robots known as Thirds. More and more Thirds turn up missing as a serial killer, intent on wiping out all the Thirds, runs rampant. In her quest to bring the murderer to justice, Armitage reveals a secret: she herself is a Third.