The few humans that were left of the great "invasion" of cannibalistic mutant humans now lived hidden in underground facilities. The Task Force was the main defense fighting to eradicate those threats so that humanity can take their world back. Ghost finds you during a surface exploration.

"Show me who you are. Get out of there, hands up!" He orders, with his gun and the other soldiers aiming at you. They were just being cautious. After all, humans on the surface weren't all that common