This is one of my all time favourite Half-Life mods. Very well designed and amazingly good fun. You play as one of the security guards, Barney.
What i liked most was the secondary option, instead of the hazard course you get "go for a walk" and you get to see what life at Black Mesa is like on an "average" day (IE no huge accident resulting in aliens killing everyone). You can do the hazard course if you wish, by taking a lift. But its much more fun to go out on duty, where you meet Kate, another security guard, and a very sexy one at that. Soon after meeting her you get to prove your shooting skills on a small invasion, if you kill them all you get "promoted".
Now on to the main game.
You start in your room, with a roomate who forgot to undress last night. You pick up your armour and handgun and head out to work. One good feature of this mod is the way H.E.V chargers are handled. You find several but you cant use them because you dont have a H.E.V (you get one later).
I must congratulate the modeler "toadman" for his/her excellent work. Gordon drives to work in a red ferrari, Kate looks very sexy and the weapons are very nice indeed.
You actually get to meet and fight along Freeman for a short time, as well as Adrian Sheperd from "Opposing Force", the expansion by gearbox (look out for a review of that soon).
The whole object of the mod is to rescue Kate and escape the Black Mesa facility, the end is particularly well done and happy, compared to many "you have died/trapped yourself in ice/been erased from time" endings that I have seen. A worthy (but large!) download for any single player fan.