Threshold is a simple shoot'em up game. You are in control of a spaceship at the bottom of the screen and have to shoot several formations of enemies at the top of the screen.
To make things more difficult you have limited time in form of a fuel tank which will get refilled after you cleared a number of waves. Besides that, your spacecraft is equipped with a laser that tends to overheat. Every shot makes the heat bar of the laser increase a bit. Once it reached its maximum you can't fire anymore and are forced to wait until it cools down while slowly running out of fuel.