Story
The game’s plot begins fourteen years after Emergence Day. Marcus Fenix, a former COG soldier, is reinstated into the military after spending four years in prison. Dominic Santiago successfully extracts him from the prison, and takes him to meet Delta Squad. The group seeks to obtain the "resonator", a device that will map "The Hollow", the underground caverns which the Locust inhabit and later deploy the "Lightmass Bomb", which will destroy the heart of the Locust forces. Fenix and his allies recover the device, but suffer multiple casualties in the process. Amidst the chaos, Fenix leads the remaining soldiers through the ruins of Sera, to a mining facility, and finally into the planet’s depths.
Delta Squad succeeds in detonating the resonator, but only to shortly discover that the device failed to map the entire tunnel network. Fortunately, they discover a larger map of the network that originates from Fenix’s old home, specifically his father’s laboratory. The group ventures to the Fenix estate at East Barricade Academy, encountering heavy 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 defeat General RAAM, before finally uploading the data. Fenix activates the bomb and both he and Santiago are rescued onto a waiting helicopter with help from Colonel Hoffman before the train plunges off a destroyed bridge and into the Imulsion below. Fully activated, the Lightmass Bomb launches into the Hollow, and eradicates the Locust tunnel networks. In the game's final sequence, Hoffman delivers a victory speech as the tunnels collapse and explode, whereupon the voice of the Locust Queen promises that the Locust will keep on fighting, and will not stop.
From Wikipedia.org