Trama

Mankind's desperate struggle for survival against a the Locust Horde, a nightmarish race of creatures that surface from the planet's core.

14 days after Emergance Day, Marcus is freed from prison and his life sentence for leaving his post during the Battle of Aspho Fields, to join the COG Army once again in humanity's epic fight for survival against the Locust. Together with his best friend Dom and the rest of Delta Squad he is sent on a mission to destroy the Locust Horde.

Just when the humans of the planet Sera attain peace after a 79 year war amongst each other, the Locust horde comes out the planet's crust to kill every human on the planet. Billions are slaughtered. In a desperate move, humans retreat to the Jacinto Plateau, the last safe place on the planet, and use satellites and chemical weapons to destroy the other 90% of the planet, and hopefully their threat along with them. When the locusts break through the plateau's defenses, Marcus Fenix, a young soldier in the elite Gears unit of the Coalition of Organized Governments (COG), disobeys orders to hold his position at Jacinto's defenses to try and save his father trapped between the Locusts and his own men. Caught an accused of deserting, he is sentenced to 40 years in Jacinto prison. 14 years into the sentence is where the game picks up, when Marcus is rescued by his military colleague and friend, Dominic Santiago. He rejoins the army, and, with the help of Alpha squad members Augustus Cole and Damon Baird, must now destroy the Locust threat and bring peace to what is left of this Romano-Greco futuristic society.—GEARSMAN

Fourteen years have passed since Emergence Day when the subterranean race "Locust" emerged from the depths of the planet Sera and attacked humankind, only six weeks after the end of the seventy-nine year Pendulum Wars between the Coalition of Ordered Governments (COG) and the Union of Independent Republics (UIR). Marcus Fenix, a former Coalition soldier (Gear) was sentenced to life imprisonment four years prior for desertion in Jacinto prison.

Coalition Colonel Victor Hoffman has pardoned and conscripted all Jacinto inmates, Marcus' long time friend Dominic "Dom" Santiago releases Marcus from his cell and the two fight past Locust forces to a helicopter escape. Marcus is drafted back into the army under Lieutenant Minh Young Kin of Delta One alongside Dom, Private Anthony Carmine and a robotic drone JACK. Their mission is to locate Alpha team who are in possession of a "resonator", a device capable of mapping the Locust tunnels and highlight key targets for the Lightmass bomb.

Delta's mission leads them outside the House of Sovereign's where a Locust sniper shots Carmine in the head. Delta finds Alpha operative Augustus "Cole Train" Cole, a former Thrashball player turned Gear at the start of the Locust war, fighting the horde alone, he is assigned to Delta and they head to Alpha's last known position, the Tomb of the Unknowns. Alpha is saved from Locust onslaught and helicopters are called in, a Locust ambush appears along with high ranking officer General Uzil RAAM who kills Kim, many other gears are slain and Delta takes refuge in the Tomb with Alpha member Private Damon Baird.

The Locust send in a Berserker, a hulking female blind monster, to hunt down Delta. Delta lure the beast outside and slay it with the Hammer of Dawn, a laser-designated satellite laser. Delta's new orders are to take the resonator to Lethia Imulsion facility ten kilometers from their position and Marcus is promoted to Sergeant. To make the long distance trip Delta must attain a vehicle, Dom offers to call in a favor from someone called Franklin, a Stranded. Stranded are scavengers and survivalists left for dead after the Hammer of Dawn strikes on Emergence Day leveled most of Sera's cities and are resentful of COG.

Dom calls in the favor on Franklin who directs he and Marcus to a gas station where it's held. Delta drive to the facility, Imulsion is a planet-wide superfuel that was the main cause of the Pendulum Wars. Locust wretches have turned lambent from Imulsion exposure and combust upon death. Delta descends to deep underground encountering a giant spider-crab Locust "Corpser" and elite troops "Theron guards." The resonator is planted and detonated while Delta returns to the surface, however the resonator failed to retrieve enough to data for a bomb launch, Baird has obtained a device which has more data and further study shows the data came from the long abandoned Fenix Estate.

Delta are flown to the proximity of the estate in helicopters which are shot down, Delta survive the crash, they find the other chopper and Cole and Baird stay to tend the wounded while Marcus and Dom head through East Barricade military academy to Marcus' home. Once there they find the Locust have overrun the place, Cole and Baird find an APC at the back of the house as a getaway vehicle, down in the basement Marcus and Dom find a lab belonging to the former's father, JACK downloads the data of the Locust tunnel network which covers the entire planet, Delta escape the lab with the data on the APC narrowly avoiding a Brumak, a giant bipedal attack Locust.

Delta arrives at a train station to catch an armored freighter carrying the Lightmass bomb, but Locust have overtaken the train, Delta fights off more Locust, only make Marcus and Dom make it onboard, while Cole and Baird catch up in a helicopter. Marcus and Dom push through enemy resistance to the front of the train where General RAAM is guarding the bomb. Marcus and Dom defeat RAAM, arm the bomb and escape on Hoffman's helicopter. The train rides over a chasm into Imulsion where the bomb arms and launches warheads targeting key Locust strongholds.

For the Coalition this is a huge victory, but narration from the Locust Queen Myrrah states that the Locust have a just cause for their war and will keep fighting.