The fact that the planet will soon be attacked by alien ships, earthlings learned in advance and managed to prepare as best they could. A powerful cannon was launched into orbit, which, according to the plan of pundits, should destroy flying saucers even on approach. In CANNON UNIVERSE you have to control this powerful cannon to stop the evil aliens from destroying the planet. Point it at every ship approaching orbit and shoot so that nothing remains of it. Don't let them launch rockets and bombs, and irradiate the Earth with laser beams. No one dares attack our beloved planet in CANNON UNIVERSE.