"Mass Effect is an award-winning, bestselling series of science fiction RPG third person shooters developed by the Canadian company BioWare and released on the Xbox 360 and Windows PC. The first game in the planned trilogy centers around a player created character named Commander Shepard and their mission to save the galaxy from a race of biomechanical beings known as the Reapers, and its followers, including the brainwashed Saren Arterius. The first game sees Shepard facing Sovereign, a Reaper left as a vanguard, who plans to allow the Reaper fleet currently hibernating in extra-galactic dark space to invade the Milky Way and destroy all sapient organic life, continuing a mysterious cycle of destruction. The second game takes places two years later, and sees Shepard tackling the Collectors, an alien race abducting entire human colonies in a plan to help the return of the Reapers. The third and final game will likely center on the final battle against the Reapers."
Source: Wikipedia, "Mass_Effect_(series)", available under the CC-BY-SA License.