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.
Incredibly lame storyline, quirky and mostly awful mapping and gameplay tactics, and hilariously bad voice acting to boot. There is still some fun to be had here however, but with many of it's cons, it outweighs the pros. Avoid this one if you can.