The story of Barney Calhoun during the Black Mesa incident.
This was an expansion pack for Half-Life and occurs after Opposing Force, which I didn't know until I heard soldiers in game talking about Adrian Shepard.
One thing I loved about this expansion is that you don't face the obstacles Gordon Freeman faced in terms of progressing the overall HL storyline. Barney's story runs parallel and has no bearing on what Gordon was doing at the time.
You do face the majority of the grunt aliens from Xen and the spec ops guys, but you don't face any of the big boys from HL. You even make it to Xen for a brief run.
This expansion adds some depth to Barney for the overall storyline as you learn he's not just some nobody who is helpless without Gordon Freeman.
This game is also filled with parallel moments where you'll see Freeman or the G-Man which is pretty cool. In fact, they reveal that Barney is seen in the opening moments of HL.
If you haven't played HL, you need to, and if you see this for $5 it would be a good pickup.