The Reapers have returned. After fifty-thousand years, they have come to repeat the cycle of genocide that has gone on since times long forgotten. Earth has fallen, and Cerberus has thrown its lot in with the Reapers, openly declaring war on the rest of the galaxy.
But when a mysterious planet appears out of nowhere in the Attican Traverse, an N7 special forces group, designation Aegis, is sent to investigate possible Cerberus activity on the planet, and what started as a routine operation turns into a war that could tip the scales in favor of either side.