On a Planet Sera,The Locust Horde attack and killed many COG Soldiers and 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 and 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,and 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,and that Locust forces were converging on that region and his squads position.

The Squad drive to the mining facility,and finally into the planet's depths. They were both ambushed. Carmine was killed and 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 larger map of the network that originates from Fenix's old home,specifically his father's laboratory. When they arrived at the tunnel,The Delta encountered the Locust resistance.

After collecting the data,the group fights their way past Locust forces and boards a train carrying the Lightmass Bomb. Fenix and Santiago battle their way through the train,and are able to kill RAAM,before uploading the data. Fully activated,the Lightmass Bomb launches into the Hollow,and eradicates the Locust tunnel networks.

Hoffman delivers a victory speech as the tunnels collapse and explode. Anya tells Marcus that RAAM was only a pawn. Marcus promises that he will find the real enemy.