This DYOM mission pack "converts" Black Mesa: Source (or Half-Life 1) to GTA: San Andreas.
You are tasked in activating the Anti-mass spectrometer, but a few moments after activating, something immedately goes wrong, alarm begins to sound, and the spectrometer cannot be shut down. After this, the anti-mass spectrometer explodes. As you wake up in the destroyed test chamber, the next chapter may start.
After all this, you know that the facility has been severly damaged. A lot of strange creatures teleported into the facility. You must fight them, and call for help.