A vast beam matrix has enshrouded the Earth. This glowing blue grid forms the Restrictor Shield, 99 sectors deep and teeming with hostile alien life. Your mission is to move through all sectors, destroying the enemy craft as you progress. Deeper sectors reveal new enemy types, each with their own method of impeding your success.
The player's ship can only move left and right, travelling along one of five beams as the blue beam matrix scrolls towards them. The ship is armed a laser lariat and torpedoes, with the laser being the principle weapon. When the fire button is pressed, the laser will shoot along the beam the player is currently on. The main aim is to destroy all white saucers in a sector, causing the Sector Sentinel to appear. This can only be destroyed with a torpedo – three are given to the player at the start of each sector – and a successful hit awards bonus points before play progresses to the next sector.
White Enemy Saucers
These are the main enemies you'll encounter. There are 15 in each sector, and you'll need to blast every one of them before you can progress. Saucers change up their attack pattern every few sectors e.g. dodging from left to right while spewing a string of bombs, or simply flying towards your ship in an attempt to ram you. Once you've destroyed all of the saucers in a sector a Sentinel craft will appear, giving you a chance to destroy it. Saucers appear from Sector 1.
Brown Space Debris
This rock-hard refuse will fly in a straight line down a beam at you. Normal laser fire will not damage it, so it can be avoided or destroyed with one of the three torpedoes you are given at the start of each life. Space debris starts appearing from Sector 2 onwards.
Yellow Chirper Ships
These mouthy aliens will announce their arrival with a twin-trill alert. They will then fly horizontally from one side of the screen to the other. Chirpers do not fire at your ship, and cannot collide with it. As such, they serve merely to provide bonus points for any player with a quick trigger finger. Chirpers start appearing from Sector 4.
Green Blocker Ships
Nigh-indestructible enemies that will streak in from the side of the screen, locking onto the beam that your ship is travelling down before trying to ram you. They must then either be avoided or destroyed with a torpedo, as they are impervious to regular laser fire. Although blockers can appear while clearing a sector of white saucers, they predominantly show up to protect a Sector Sentinel. On later levels, more and more green blockers will assault your craft, attempting to thwart any effort to blow up the Sentinel by blocking your torpedoes and causing you to switch beams to get a clear shot. Blockers start appearing as regular enemies from Sector 6.
Green Bounce Craft
This rotund enemy comes from the side of the screen and bounces horizontally across from one beam to the next, hoping to land on your ship to destroy it. If it is unsuccessful, it will bounce off the opposite side of the screen to return at another random interval. Regular lasers cannot harm it, so shoot it with a torpedo or try not to be on the beam it bounces on. Green bouncers appear from Sector 7.
Chargers will zip down the screen at you along a random beam. They have two methods of attack, depending on what beam your ship is travelling on. If you're on the same beam as a charger when it reaches the bottom of the screen, it will collide with your ship and destroy it. If you're not on the same beam, the charger will wait for a few seconds before leaving the screen. During this small pause there is a chance that you will move sideways and onto the beam where the stationary charger is lurking, resulting in your death. While they aren't actually destroyed by your laser, firing a quick shot up the beam will cause a charger to bounce backwards into the distance from where it came. Chargers begin to appear from Sector 10.
These devilish ships will track onto the beam your ship is flying along and attempt to ram you. Unlike green blockers, however, trackers will change beams to follow you should you try to avoid them. Initially, they will only change beam once to try and cause a collision. However, the further into the beam matrix you travel, the more intent the trackers will become; soon they will start to change course two or more times in their efforts. Additionally, they are invulnerable to common laser fire, meaning you'll need to be fast to avoid them or see them off with one of your precious torpedoes. Trackers start appearing at Sector 12.
Red Zig Bombs
The final enemy to appear is one of the trickiest. Like the chirper, a Zig bomb will make a sound before making its way onscreen. When it does, it will find the beam your ship is on and start to come towards you. Zig bombs cannot be killed outright with regular laser fire, but they can be handled in various ways. If you manage to shoot a Zig bomb with your laser, it will change from red to green. It will then continue its flight down the beam it has chosen, before disappearing off the bottom of the screen. If, for whatever reason, you don't shoot the Zig bomb, it will travel down the beam before performing a last-minute flip across one beam towards your ship. This means if you fail to shoot the Zig, you should ensure that you are at least two beams away from it before it leaves the screen. Of course, the third method of taking a Zig out is to blast it with a torpedo. Zig bombs will appear from Sector 14 onwards.
Once all of the white saucers in a sector have been eliminated, an alarm will sound. The Sector Sentinel will then slide into view from either side of the screen. It will continue across the beam matrix horizon until it is either destroyed or leaves on the side opposite its starting position. During its migration, you can attempt to bring it down with any torpedoes you have in your stock. The higher the sector number, the more green blockers will flood onto the screen to prevent the Sentinel from being destroyed. A successful hit will cause the Sentinel to explode and bonus points are given depending on the number of lives you have accrued. Whether or not you destroy a Sentinel, the sector has been cleared and you move onto the next one.