On day an alien race of slugs is out patrolling space, when they encounter the Lucasarts logo. The logo, as we all know looks like a statue of a person with his hands extending towards the sky. Seeing this logo, the slugs determine that ours is a race made up of BONES and of ADVERTISING, two things the slugs simply hate. So they attack.
With the slug invasion in full swing, earth is quite simply outnumbered by the gooey slugs. It must recruit special super soldiers, Big Sky Troopers, armed with a special heavily armored suit to combat the menace. So urgent is the need for qualified Big Sky Troopers, that recruiting and rank are petty things. It is not unknown for example for a brand new trooper to start as a 21-star general in the army.
Once selected, it is up to your Big Sky Trooper, piloting a FIDO class starship, to engage the slugs in space and in planetfall and thwart their plans to destroy mythical galactic devices and conquer humanity.