"Elias, Helene, and Tavi are Fivers - in training to become elite military recruits for the Martial Empire at Blackcliff Academy. When they're sent on a dangerous mission to steal a heavily-guarded poison on Isle South, the soon find themselves up against surprising enemies - including other recruits. As the true horror of their assignment is revealed, these three will begin to confront the harrowing realities of Martial rule, their place in the system...