The original trilogy largely revolves around a soldier named Commander Shepard , whose mission is to save the galaxy from a race of powerful mechanical beings known as the Reapers and their agents, including the first game's antagonist Saren Arterius. The first game , released in , sees Shepard investigating Saren, whom Shepard slowly comes to understand is operating under the guidance of Sovereign, a Reaper left behind in the Milky Way tens of thousands of years before, when the Reapers exterminated virtually all sentient organic life in the galaxy as part of a recurrent cycle of genocide for an unknown purpose. Sovereign's purpose is to trigger the imminent return of the Reaper fleet hibernating in extra-galactic dark space, restarting the process of extermination. The second game takes place two years later, and sees Shepard battling the Collectors, an alien race abducting entire human colonies in a plan to help the Reapers return to the Milky Way. The final game of Shepard's trilogy centers on the war being waged against the Reapers.