You must destroy a giant alien being in orbit around the earth ... from the inside!

The game is a shoot-em-up, where you explore 16 maze-like levels, shooting various enemies that live within the alien. There are various objects to help you, including different weapons (effective against different enemies), and a map of each level.

You must destroy 4 of the alien's organs, the kidneys, heart, lungs and finally the brain, that appear on the 4th, 8th, 12th and 16th level respectively. However, to destroy each organ you need to find the 3 parts of a weapon that can bypass that organ's defences.

The levels are interconnected, and although progress is mostly from level 1 to level 2, etc, some backtracking may be needed to find all the weapon components.