The many underground caverns to the Doombase have been invaded by aliens. To get rid of them you have to collect all the keys required to build a Doomsday device. The keys are scattered throughout the numerous caverns and are defended by alien mercenaries.
Your equipment consists of a full space suit and back jet pack which will help you traverse these dangerous caverns. You also have a small supply of bombs with which to destroy your attackers if you can.
Find your way through all the caverns to ensure that you collect all the keys before the Doombase can be saved. Extra ammunition can be collected on your travels.