Enemy Within's single-player campaign starts the same way as Enemy Unknown -- bunkered in an underground military base at the brink of the end of the world, following the escalating war with the aliens to its climax. Think of the expansion as a lateral addition to a core game and mission list that remains unchanged. It's possible to play Enemy Within almost exactly like Enemy Unknown, but Enemy Within adds an optional quest line early on: Defend the XCOM nations and XCOM itself from a transhuman threat... an organization called Exalt.
To further humanity's evolution, Exalt attempts to disrupt XCOM operations and promote the assimilation of alien technology into human biology. Thick with narrative irony, XCOM simultaneously develops two tracks of transhuman upgrades, the exact kind of alien-human integration that Exalt wants: MECs and gene mods. Each unlocks after the construction of cybernetics or genetics labs, respectively, enabling the manufacture of XCOM super soldiers to combat both Exalt and the Sectoids' breed of elite soldiers.
