Defend the galaxy through ten waves of alien ships. Wave 5 and wave 10 end with a huge boss ship. Your ship fires by itself, so all you have to do is dodge the enemy fire and line yourself up.
On a phone: put your finger on the screen and drag. The ship follows your finger, a little above it.
On a computer: hold the mouse button and move, or use W A S D or the arrow keys.
Destroyed enemies sometimes drop power-ups: G adds more guns (up to three), S gives a shield for six seconds, R makes you fire faster for eight seconds and H gives an extra life. Getting hit costs a life and one gun level.