This mod takes place during the Black Mesa incident. You take on the role of a HECU Marine as you fight your way through different sections of the facility and carry out varied missions. *This is not a rebuild of any game, but features areas from Half-life, Blue Shift & Opposing force in the Half-Life fashion.
It's a really great mod, with lots of puzzles and etc. Problems are the very cheap situations and glitches, but then again they will be fixed later on.
Overall, I recommend it!