Some time into a doubtful future, man has looked far through the depths of space to discover a strange series of constructions linking our space to that of another dimension. These alien artifacts are tunnel-like tubes, through which the hostile forces of another galaxy threaten to pour, destroying all before them. The combined resources of seven planets have built a single craft capable of carrying one man on a desperate mission to stop the building of alien tubes, sealing the aliens in their own universe forever. The mission is to fly the craft round the rim of the tubes, clearing each wave of aggressors until the pilot can blast his ship down the tube to the next section. Only when all the tubes have been cleared and destroyed will the universe be safe!