The player assumes the role of captain of a battle starship, charged with defending the frontiers of space from hostile alien ships, which come in two sizes — large and small. The larger ships have cloaking devices which make them invisible. Damage is inflicted on ships when they are shot or rammed by other ships. Uniquely for the time, the player vessel remains locked in the centre of the screen and the world rotates and moves around it. Player shots also have limited range and are subject to the same rotation as everything else, which means the player needs to predict where shots will need to go in order to hit the enemies. The ship has both long range scanners and shields but may only use one at a time. By default, the game will automatically switch between them based on the proximity of enemy vessels but the player can optionally take full control. Using shields, thrusting, turning and firing all use up energy which is also depleted when hit by enemies. Energy will automatically replenish, most quickly if the shields are off and the starship is not moving but obviously this leaves the player open to attack.