The galaxy is at war.

Striking from beyond known space, a race of terrifying creatures called the Reapers has begun their destruction of all sentient life forms. The Alliance, led by Commander Shepard, is the only hope for saving the galaxy. He must rally the civilizations to launch one mission: find the Catalyst to activate the Crucible and end the Reaper threat once and for all. Meanwhile, a clandestine operation known as Cerberus, led by the illusive Man, seeks to take control of the Reapers to make humanity the dominant force in the galaxy.