You begin by finding yourself stuck at the bottom of a cave with no idea how you got there. Befriending the first thing you see, an intelligent gun, you begin your ascent to freedom.
Control your character with AD or Arrow Keys, jump with W/Space/Up, and use the mouse to aim and shoot (or switch control schemes in the options). As you acquire more guns, use QE or 1-4 or the mouse wheel to switch between them. Open the menu/upgrade screen with tab, escape, or P.
In the options menu you can mute the music and/or sound, change the text speed, and change the control scheme.