You play as Thomas Redgrave. A scientist working in Area3 Research labs. The story is parallel with the events in Black Mesa, the same day when Gordon Freeman and Scientists in Sector C started the whole event.
I'm surprised by all the high votes here: it wasn't THAT good. It was very mediocre. The initial part during the Tram Ride and in the research labs was quite good, but then it goes downhill. Bland level design, easy combat, an Axe instead of a Crowbar (The Crowbar looks better). Also you mainly visit the Ventilation/Sewers and that's disappointing. At least you visit some familiar areas. When you begin fighting on the Surface it becomes a little better, especially the tank part! And the ending was quite good, too bad he didn't escape. But the the maps before the Surface deserve a 6. Also, weird Voice Acting but that's not a big problem!