Though divided, the Human Sphere is strong, and few among it stronger than the O-12 Council's own Commander Jonathan Shepard, the Lion of Paradiso. As a potential candidate for the Citadel's Spectre unit, Shepard must first prove his ability by solving the death of one of the Citadel's top agents, uncovering a shadowy conspiracy which will change the galaxy forever...

The UNSC and Covenant separatist began their final assault on the last citadel to decide the fate of both warring factions. However they were not alone an unknown entity force soon makes their way through the lines vigilantly with opposing fire-power. The question still stands what is their true objective here on this Ark.