It's 1975; you've been called back to active service with the army. Enemy soldiers have captured an important scientist, and you must infiltrate Vietnam to bring him back. In typical military intelligence, you and another operative have been sent in with nothing but the fatigues on your back and a gun.
Imagine playing Operation Wolf without a gun -- that's what Nam-1975 is all about. The game is a 2D scrolling shooter, except you don't have to move to keep up with the scrolling. The screen is divided into the foreground plane, along which your soldier can move left and right, and the background plane, where the enemy soldiers are. You must run along the foreground plane dodging enemy bullets and grenades while shooting them up yourself.
Your soldier has a crosshairs corresponding to where he's aiming. When your soldier moves left or right along the foreground, the crosshairs moves with him. Pushing up or down on the joystick causes the crosshairs to move up or down. If you hold down the shoot button, you can stand in place while aiming the crosshairs.
At the beginning of the game, you have only a regular gun with infinite ammunition and a limited supply of grenades. Items can be acquired by shooting enemy units and buildings. Most of these items are point bonuses, but some replenish your grenade stock or give you special weapons like flame guns or machine guns. These special weapons have limited ammunition, and your soldier returns to using his regular gun once that ammunition is depleted.