You play in the role of Captain Percy Freeman, a Special Forces officer who is assigned the task of tracking down and assassinating the renegade Colonel Kurtz. Things take a turn for the worse when, having set off on your mission, the higher-ups in the chain of command mysteriously decide that they want you dead, while you discover to your horror that someone has been breeding an inhuman master-race of hideous undead mutants which is now rampaging across the jungle slaughtering everything in its path. Stuck between the Viet-Cong, the U.S. Marine Corps, and the hordes of mindless zombies, with only your sidekick Barney for company and your trusty chainsaw for a weapon, your one remaining hope is to keep going deeper and deeper into enemy territory, wading through the bodies of the fallen, towards the elusive Kurtz...
Well if you have played the original version then all I need to say is, it's pretty much the same but with more smoother combat and better models (with some extra scenes here and there).
However if you haven't played this mod before then I recommend you to stop what you doing and download this right now (but only if you own HL:Source or HL1 for the original version), this mod has so much content in it that it took be it's own game (only a small amount of content from Half-life 1 and it's mostly just models that has been reskin). The mod takes place doing the Vietnam war and your mission is too kill a rogue colonel however something more evil approaches, the story is really good and the locations you go through are unique (you never go through the same locations as well, always something new for each chapter). This mod also houses lots of new weapons to use on enemies and alot of them are fun to use, the only problem with the enemies is that some of them are reskins of HL1 enemies however they do have some new things added to them. In the end I don't want to spoil too much, so all I going to say is if you want a more smoother version with some extras of this mod then download this Source port however if you want a more original version or don't have Half-life:Source then download the Half-life 1 version but beware that the AI followers are annoying (the only problem with the original one)