Before the game will take commands, the border right next to the game screen needs to be clicked by the mouse.
There are 8 red reactors on screen that will become active and turn green at random intervals.
Use keys 1 - 4 & 6 - 9 by using the top numeric keys or the numpad to shoot the reactors to shut them off.
Once the "5" in the center turn blue the player may choose to press the "5" key to increase the difficulty and their point multiplier.
If all 8 reactors become green it's game over.