A gargantua has escaped from a lab, where they are trying to find his weakness. He was injured by a marine,which was made short work of, and he was trapped inside a illusio-pit. Your job is to find it, and kill it.
By the way, you can rename the ".zip" portion of this file to a ".pk4" and it will load without extraction if you are playing with FreeHL (similar to custom maps in Quake III and DOOM 3).