Your job, obviously, is to get out of the lab and find help. You just have to get through all the creatures and on top of that, get through the military who has been alerted to the disaster that has just taken place. There are two reasons that Half Life was so successful. First, it is one of the only first person shooters that mimics a REAL-LIFE environment. Everything seems plausible to a certain degree.
Quake comes closest but really had no story line at all.
In Half-Life, you cannot finish the game without understanding the storyline and plot. There were times that I actually jumped in this game. Any time you can get that kind of emotion out of a video game, it means the game is doing its job. The only other game that has freaked me out like this one was The Pandora Directive. With EAX, the game incorporates audio reverb to make the effects ever so realistic.