On the Planet Sera,The Locust Horde attack & killed many COG Soldiers & civilians,declaring war against humanity.

14 Years after Emergence Day(E-Day),Marcus Fenix,a former COG Soldier,is reinstated into the military after spending 4 Years in prison for deserting his military post in order to make a vain attempt to save his father,Adam. He was extracted by his friend,Dominic "Dom" Santiago along with the Delta Force,Anthony Carmine & Minh Young Kim. They received order by General Hoffman to secure the resonator,a device intended to map the Locust underground network out for the Lightmass Bomb.

Marcus met the Alpha Squads members,Augustus Cole,Damon Baird,& Gyules. While securing the resonator,Gyules is killed by General RAAM. They manage to escape. Back in Headquarters,The Squad met with Anya Stroud,who informed Marcus that Lethia Imulsion Facility,where an entrance to the hollow was located,was ten clicks away,& that the Locust forces were converging on that region & his squads position.

The Squad drive to the mining facility & finally into the planet's depths. They were both ambushed. Carmine was killed & Kim was wounded. As the Locust forces closes in,Kim sacrifice himself by detonating the resonator,but the device fails to map enough of the tunnel network. They discover a large map of the network that originates from Fenix's old home,specifically his father's laboratory. When they arrived at the tunnel,The Device encountered the Locust location.

After collecting the data,the group fights their way past Locust forces & boards a train carrying the Lightmass Bomb. Fenix & Santiago fight their way through the train & are able to kill RAAM before uploading the data. Fully activated,the Lightmass Bomb launches into the Hollow & eradicates the Locust tunnel networks. Hoffman delivers a victory speech as the tunnel explode. Anya tells Marcus that RAAM was only a pawn. He promises that he will find the real enemy.