In an all-out attempt to save humankind from extinction at the hands of the Locust Horde, Marcus Fenix and Dominic Santiago, childhood friends and soldiers, are ready to fight in another battle side-by-side, but the return of Dominic's brother Carlos, and a great secret, threatens the bond of their friendship.

Marcus Fenix and the Gears begin rebuilding human society in the wake of the Locust Horde's defeat, and while they can handle the raiding pirate gangs, they are not prepared to face a menace that even the Locust's feared.