Half life... Your mission begins when a scientific "breakthrough" goes wrong. They have opened a portal to another world, in which, evil aliens are attacking. When you start you have no weapons at all. You are just punching but soon you will find a crowbar and your first gun. The list of guns goes as follows:
Crowbar
Pistal
Magnum
Alien Arm
Shotgun
Machine Gun
Crossbow
Rocket LauncHer
Orange Laser Device
Blue Laser Device
Grenades
Laser Det packs
Satual Charge
Alien Creatures
Once you beat your world you go to stop the aliens at their source! Then you beat the boss and beat the game. I have beat the game and it is really fun.(save before each battle so you can play it multiple times to get the best ammount of health) Have Fun!