>>717767984
It's an expansion for Half-Life, set during the main game only you're one of the Marines sent in to stop Freeman and cover up the accident that caused an interdimensional invasion. You don't get these orders though because your transport crashes as your CO begins telling you this.
Think of it as a briefer, faster and more action packed version of the main game. You even come across scenes from the main game from a different perspective, which is cool.