From the game's manual and the back of the package:
Centuries ago, a Hyperion military base was established halfway through the galaxy's only known express route. All ships using this route are forced to stop and pay a very high toll. The penalty for non-payment? Instant death! All attempts to destroy the base and stop this ancient injustice have met with dismal failure.
Your Assault Ship was specially designed to destroy Hyperion Military Bases. Your mission is to destroy as many of them as possible. You can fire missiles through the gaps in the Force Field or from inside the Force Field. If you are able to sneak inside, you must quickly destroy the Base before the Force Field closes in on your Assault Ship. This is no easy task! The Base can construct electrifying Shields during battle and is protected by a variety of deadly defenders. If you are able to avoid all of them, you will be truly deserving of the title Space Master X-7.