You play as a research associate, Gordon Freeman, who works in the Black Mesa Research Facility (which is very similar to the real-world Sandia Labs). One day, when the scientists are doing an experiment with some anomalous materials a gate opens to an alternate dimesion known as Xen. Then all hell breaks loose; hundreds of aliens start to teleport into our dimension, quickly overrunning the facility. To make things worse, the government sends troops intent on killing both the aliens and the workers of Black Mesa, to prevent anyone from learning of their secret projects. As Gordon makes his way through the complex with the help of Barney the security guard and some scientists, he discovers that there were already aliens in the facility, brought back for observation by the science team studying Xen. The scientists tell him that their last hope is in the Lambda complex. When Gordon arrives, it is revealed that he must teleport into the alien dimension and kill the giant Nihilanth, the entity that is keeping the portal open. This creature is the last boss. After that, the mysterious character known as the G-man offers Gordon a permanent job... in a tram surrounded by stars. If you reject him, you die. If you accept and walk through the teleporter, he says you've chosen wisely and that is where Half-Life 2 starts off. So, presumably, you work for the G-man in Half-Life 2.