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 2007, 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 50,000 years before, when the Reapers exterminated all sentient organic life determined to have met or exceeded a threshold of technological advancement 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 centres on the war being waged against the Reapers. The fourth installment takes place in the Andromeda Galaxy and features a new cast of characters.