The game's goal is to defend the United States against nuclear attack, much in the style of a less frantic Missile Command.

As the game begins, the iconic line "Greetings Professor Falken:" appears on the screen, quickly followed by, in a box: "Select a Challenge Level from 1 to 8 for Global Thermonuclear War".

Defenses consist of ABMs, interceptor aircraft, submarines and an experimental particle beam satellite that trumps everything else but weaves in and out of sectors on a fixed orbit.

Enemy presence and the destruction of bases and cities lower a sector's Defense Condition (DEFCON) status.

If it stays at 1 for 60 successive seconds, or if at any time all cities and bases have been destroyed, an automated counterstrike triggers global thermonuclear war and the player loses the game.