You didn't exactly have a time limit, but the more time passed, the harder aliens arrived, starting with one-man(alien?) scouts to do Roswell-grade pranks to Harvester grade ships to abduct and mutilate cattle, to fuckhuge battleships that eventually started attacking your base. It falls upon the Extraterrestrial Combat Unit, or the eponymous X-COM, a multi-national task-force, to deal with the invasion by blasting the fuck out of the Aliens, stealing their shit, researching their technology, and using it to fucking kill them off, all the while fighting off terror attacks and trying to figure out where in Uranus these fucking things are coming from (as it turns out, from Mars, and if you win, the final mission involves you going to Mars and dropping an Exterminatus on their asses). Aliens are arriving, and they are most assuredly hostile.
