Far in the future, the peaceful Confederation has transcended violence and evolved beyond war. Unskilled in the art of combat and vulnerable to attack from the relentless Others, the Confederation enlists humankind to fight alongside several other warlike species in their defence. Her latest mission sees newly promoted Gunnery Sergeant Torin Kerr escorting a recuperating major and his doctor to Crucible, the Marine Corps training planet. But when the troops are suddenly attacked by their own drones, Kerr is caught in a desperate fight to protect the major, his doctor and a platoon of untried recruits.