Waterworks addon - Half-Life 2: Episode Two

The end of the war for City 24 is drawing near. Most of the streets are taken over by the Resistance but the heart of the city is impenetrable for the rebels, a massive Combine stronghold surrounded by tall steel walls.

If the siege isn't quick enough, a large reinforcement will arrive to aid the besieged and can possibly even take back the city. Fortunately, a hidden Combine depot was found deep underground near the waterworks facility that is directly connected to the stronghold.

You are a part of the strike team tasked with capturing the vital depot. You were ambushed by the Combine forces, leaving you the sole survivor. Now it's up to you, and only you, to finish the mission or perish in the uneven struggle.