Half-Life: Intolerable Threat (2002)

As Gordon Freeman, you must face down the Administrator for the last time. The Administrator has taken control of HANGAR 16 a government command and control facility in the desert. From here he is possibly planning a coup against the government of the United States, and vengeance against those who have opposed him in the past. You need to meet with an agent who has the access codes to enter the heavily guarded base, infiltrate it and prepare the way for friendly forces to attack.