All parts of the Mercy series.

Story: You are a survivor in the ruins of a city the Combine deemed too difficult to govern to its tall buildings and hiding places.
And so, the Combine has showered headcrab cannisters into the area in case anyone was left from either the 7 hour war, or has entered the zone for a better but harder life.
The Combine do monthly sweeps of the area to look for any survivors from the headcrabs they already drop down.
Soon, in a couple days, the Combine will begin one of their many sweeps, and you must escape before they do.