The Gears of War series takes place on a fictional Earth-like planet named Sera. The planet had a history of conflict that took Sera to the brink of destruction. This shocked the people into a rejection of their destructive ways, leading to a golden age. Although most conflict was solved peacefully, civil liberties and crime remained as troublesome social issues. A golden age of culture, science, and the arts ensued, yet one problem stood prominent above the rest: energy. Sera's sources of petroleum and nuclear fuel were not sufficient to meet the global demand. Imulsion - with its promise of near limitless energy - was thought to be the solution to mankind's great need. Humans discovered liquid Imulsion, which flows underground, and became a key power source. The value of Imulsion led to a seventy-nine year war over the substance, until the Coalition of Ordered Governments (COG) was formed to put a stop to the war. Shortly after peace settled, however, humans were attacked across the planet by the Locust, a race of beings that dug their way out from underground. Much of humanity was wiped out during the first Locust attack, otherwise known as "Emergence Day," and the remaining humans gathered their numbers on Jacinto Plateau, a mass of hard rock the Locust could not burrow through; the humans that could not make it became known as the "Stranded," and took shelter among the ruins. Out of desperation, the COG employed a "scorched earth" policy by firing orbital lasers across the surface of the planet to deny the Locust any spoils of war. The COG forces and the Locust continued to battle, each side desiring the complete destruction of the other.
The campaign mode of the series has focused on the character of Marcus Fenix, a COG soldier formerly imprisoned for abandoning his postduring the early Locust attacks to save his father, Adam Fenix, a key scientist studying the Locust. At the start of the first game, Marcus is rescued from an abandoned prison facility by his best friend, Dominic "Dom" Santiago, who had lost his wife Maria during the Locust attacks and has been looking for her since. Marcus, after his rescue, is readmitted to the COG force and promoted to lead Delta Squad. They are later joined by Augustus Cole, a star "thrashball" player before Emergence Day, and Damon Baird, the squad's mechanic.
Most of the first two games take place underneath the surface of planet Sera in a vast system of tunnels known as The Hollow which were dug by the Locust. What makes the existence of these tunnels deadly to the inhabitants of Sera is they allow the Locust to move any unit in their army, ranging from tiny little explosive units called tickers to the giant walking tank-like behemoth called the brumak, throughout the entire planet. The other problem with the Locust living in the Hollow is the COG’s only fuel source, imulsion, flows like an endless river in the depths of the Hollow.
Throughout the games you will also play in very close quarters environments like old abandoned cities and facilities. In these areas Delta is often split into two groups of two. When split into these groups each time will be different. It may be that the two playable characters are a group or the two playable characters may split up and go with one of the other Gears. If the latter happens then you can be sure that there will be two different objectives for each group, but you will still be given a chance to help the other player. This is where that close quarters comes into play because one of you may be up higher than the other so you will have to help the other take down enemies from above