In XCOM: Enemy Unknown the player is called upon as a commander, to lead an International organization, known as the XCOM project, against an alien invasion by mysterious forces.

Along the way, the player researches technology gained from the invaders and fashions it into more powerful weapons to counter increasingly powerful enemies.

In the Enemy Within expansion, a terrorist organization known as EXALT appears, who support the aliens and sabotage the XCOM project's work, stealing money and setting back production. The expansion also makes avaliable new weapons and gear, as well as adding new enemies.