The fight against the Combine begins
Set between the events of Half-Life and Half-Life 2, Alyx Vance and her father Eli mount an early resistance to the Combine's brutal occupation of Earth.
The loss of the Seven-Hour War is still fresh. In the shadow of a rising Combine fortress known as the Citadel, residents of City 17 learn to live under the rule of their invaders. But among this scattered population are two of Earth's most resourceful scientists: Dr. Eli Vance and his daughter Alyx, the founders of a fledgling resistance.
Source: The Game Website.