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.