From the Back of the Box:
"When Operation Thunderbolt confronts a deadly alien force there is only one way to survive... check out SPACE GUN.
Rescue the crew of an Earth-bound cargo ship hijacked by an unknown alien life form. Annihilate these hideous creatures throughout all levels of your space craft using your high powered weaponry. There is a stunning arsenal of 'specials' at your disposal, such as the 'freeze' and the 'blade' plus many more. From first person perspective you confront an interminable onslaught... it is the survival of the fighters.
Space Gun gives you the power... have you got the guts?"