You are Gordon Freeman. On your quest to the Lambda lab you find another one of Black Mesa’s nuclear weapon storage facilities. This one is different though; a rogue group of military has stormed the building and plans to detonate the weapon to rid earth of the alien invasion. You, becoming aware of this, must deactivate the nuclear device to save innocent scientists, and to prevent the destruction of Black Mesa’s vast amounts of data, research and information.
It hasn't aged well, especially in an age where lack of details can kill a mod, but I remember this fondly from the old days of Half-life, so it may just be nostalgia talking :/
However, any fan of half-life shouldn't give this the cold shoulder if they want some good ol' half-life fun. :)