From the back of the case (EU Mega Drive edition)
"Cats, the leader of a band of swashbuckling space pirates, is terrorizing the galaxy. You must pilot the only remaining Milky Way attack craft into combat and stop Cats and his buccaneers!"
The game gained Internet notoriety with the European port, in which the translation was somewhat lacking: "All your base are belong to us" being the prime example.
Source: Wikipedia, "All_your_base_are_belong_to_us," available under the CC-BY-SA License.