You discover that alien technology relies on Elerium-115 , a stable heavy element not found on Earth, which becomes the most vital resource for humanity's advanced crafts and weapons.
As the Commander, you start with basic human ballistics while fighting various alien species, such as the physically frail but psionic and the brutish Mutons . The "story" is largely driven by your research:
Destroying the Alien Brain severs the link between the invaders and their leadership, causing the alien forces on Earth to collapse and saving humanity. UFO: Enemy Unknown
The story continues in X-COM: Terror from the Deep , where a second wave of aliens awakens from the ocean floor.
Interrogating high-ranking alien captives reveals that the various species are being manipulated by a master race. The Revelation You discover that alien technology relies on Elerium-115
Once X-COM develops the (a powerful interstellar craft), they launch a final mission to Mars.
Soldiers must fight through the Martian surface and into the underground alien complex. The story continues in X-COM: Terror from the
The modern XCOM: Enemy Unknown reboots this story for a new timeline. In the timeline of XCOM 2 , it is revealed that X-COM actually lost the war early on, and the "victory" from the previous game was a simulation run in the Commander's mind while they were held captive.