This is the second part of HECU Corporal Timothy Collins story and another sequel to Half-Life: Operation Rosenberg.
After completing Operation Rosenberg, Corporal Collins gets back to headquarters where he is assigned to a new mission. The nuclear explosion took place at Black Mesa and a short time later, a dimensional rift was opened in a nearby town. Aliens invaded all the area.
The Marine Corps sent troops to contain the extraterrestrial plague. Collins mission will be to kill all the creatures and find the way to seal the rift.