Once again, Earth is under attack by alien invaders. As the pilot of a laser-shooting fighter that can maneuver along the bottom 1/4th of the screen, you alone must ward off 24 squads of swooping attackers. You begin the game with four ships and will win an extra one for every 50,000 points you score. Each ship is equipped with one Hyperdrive, which temporarily enables you to move so fast that the alien attackers will appear as though they are moving in slow motion.

Fuel is a precious commodity in Threshold, so you should destroy each enemy squad as quickly as possible. After you knock out four squads, the mother ship will come to refuel your fighter. Any leftover fuel results in bonus points. Along with fuel concerns, you must also keep an eye on your laser indicator. If you sit on the fire button continuously, your lasers will overheat, temporarily putting them out of commission.

Take Demon Attack for the Atari 2600 and add a fuel gauge, a laser indicator and a Hyperdrive, and you'll have something similar to Threshold. ~ Brett Alan Weiss, All Game Guide