X-COM: Apocalypse

Half a century after the end of the second X-COM campaign, the last battle of T'leth has severely damaged Earth's biosphere. As a result, several self-contained Megalopolis-type cities were proposed to provide habitation for humanity. The game follows Mega-Primus, the first of these cities, built over the ruins of Toronto, Canada. Meanwhile, the off-world colony of Mars is exploited by the Elerium mining corporation, Solmine, and oppressed by MarSec (MARs SECurity).

The alien threat in the game...