Previously on Gears of War...
Gears of War begins 14 years after Emergence Day. Marcus Fenix, a former COG soldier, has been imprisoned for 40 years after abandoning his post in a failed attempt to save his father, Professor Adam Fenix. After serving only four years, Fenix is reinstated as a COG soldier (Known as Gears) and is immediately thrust back into the fight.
While many show animosity toward him for the crimes he was convicted of, Fenix is placed in Delta Squad comprised of former squad mate Dominic Santiago, squad leader Lt. Minh Young Kim and rookie Anthony Carmine and quickly gains back the respect of his peers.
The COG plan is to use a device called the "Resonator" to map the series of underground tunnels used by the Locust Horde and drop a "Lightmass Bomb" in an area that would be devastating to the monstrous beings.