Set in the United States in 1962, the game tells the story of the Federal Bureau of Investigation (FBI) special agent William Carter, tasked with investigating and stopping an unknown extraterrestrial force that is threatening the very existence of human kind.

The game is set in 2035, 20 years after the events of XCOM: Enemy Within, in a scenario in which humanity was defeated by aliens. XCOM proved incapable of attenuating the alien threat and was quickly betrayed by the council nations, who surrendered to the aliens shortly after the invasion began.

The game, set in the modern-day Earth, deals with an international response to a global alien invasion. The player assumes the role of the commander of XCOM, the most elite military and scientific organization in the history of mankind, engaged in a seemingly hopeless war against an unknown extraterrestrial enemy with far better technology and capabilities.

XCOM: Enemy Within is a stand-alone expansion pack for the strategy video game XCOM: Enemy Unknown.