Aliens have invaded a subterranean mining colony. The player must fly their prototype ship (only named as an "Attack Ship", or "Experimental attack fighter") through caverns to destroy the aliens and rescue trapped miners (throughout several of the levels) without running out of fuel, kind of making this, at times, a cross between Time Pilot and Gravitar (although the miners the player rescues are on the ground, rather than floating through the air like with Time Pilot, and the difficulty for this game can be considered high like with Gravitar, as the player's ship is constantly running out of fuel). The ship is highly maneuverable and fires several shots in rapid fashion, along with having a Mega Shot that will charge over time when not in use.
There are also capsule power-ups that change color that will improve the player's weapons; the player must touch the module at the right time to get the color power-up that they want for their ship, since, if the player keeps on consistently collecting the same color power-up throughout the game (i. e. such as the blue power-up every time), then their weapon's level will increase in power each time, up to a level of four maximum. The power-up colors are red, blue and green, and the player's ship starts each game as Red Level 1.
There are also modules scattered around various levels for missiles (10 in each module), extra lives, upgrades for the player's ship for later underwater levels, and other items for the last few levels that are required in order to beat the game.
Each time the player's ship collides with an alien being or force, cavern wall or any other obstacle, along with getting hit by a weapon, their shield will take damage, which is represented by a red indicator on the ship icon that is currently in use at the bottom left of the screen. Once the player's shields have taken too much damage the player will lose that ship and the game will end when the player has no remaining reserve ships. The shields will recharge themselves over time though if they take no damage, along with if there is a shield recharge station on the level that the player is currently on that they can dock with.
Each mission has a different set of objectives, which are outlined at the beginning of every new level, which are shown during briefings, gathered by an instrument known as the SatScan. Once a level is completed, the player receives a bonus for time, miners rescued (if applicable), and lives remaining. If the player fails a mission then they will have to restart it and their fuel level won't be replenished.
Here the player must rescue a stranded spacefleet spy group and recover the first of several Sub-Modules so their ship can get through later underwater levels.
Items and enemies to be found in this mission are extra life and missile modules, a hidden power-up (all colors) and missiles, moving platforms to access additional areas of the level, and various alien/robot/gun turret forces. (Note: fuel modules are found on all levels.)
Here the player must retrieve another Sub-Module, although it is in a cave which is sealed, which the player has to defeat a creature (which is a giant head with three faces) in order to open the cave. Several men are also trapped in the cave and have to be rescued. There are rail transports here, which, once the Attack Ship attaches onto one, it will save the player fuel by moving along the rails and not flying and burning fuel.
An extra life, missiles (two of those), a module for red, green and blue weapon power-ups, and a shield recharge module all adorn this level. The moving platforms and one of the enemy forces (a robot with long arm appendages) return from the first mission, along with other forces that jump up and down great distances that can smash the Attack Ship into a cavern wall.
This mission has a high-powered mining laser, which there are several deflectors that the player has to use to defeat it, along with employing a utility truck that will assist the player, as the equipment the player collects can be dropped down to the truck. The player has to destroy some of the alien housing to get to one of the deflectors which will be used against the laser itself. Also a Sub-Module and men have to be rescued on this level. Missiles and shield recharges can also be found, and enemy forces include a triple gun turret returning from level one and ships that fly around above the laser that are indestructible.
The player needs to destroy the enemy leader on this one (only identified as "The Thing" during the SatScan briefing). Again a combat squad has to be rescued, along with the player having to chip away at a building structure to retrieve another Sub-Module. There is also a hidden extra life in another structure and hidden missiles as well, along with a module containing all weapon power-ups.
Enemy forces include the return of the punching robots from level one, tanks, and what could be acid that drips from the cavern ceiling that damages the Attack Ship's shields.
Aliens have sabotaged a nuclear power plant, which is about to explode, so the player must destroy the power plant before its fusion reactors overheat. An anti-shield fire module must be located, then used to destroy the power plant (as the player's regular firepower won't do it); once it's destroyed, the atmosphere will be contaminated, so the player must rescue the workers. The reactor core from the power plant must also be dropped on The Saboteur (a giant crab-like creature), since the Attack Ship's regular weapons are useless, and that frees up the Sub-Module the saboteur is guarding.
An all shot power-up module is available, along with several returning alien forces (the punching robots, tanks), and a couple of new airborne enemies.
This site provides most of spacefleet's fuel. Aliens have activated a guardian robot to try to cripple your forces. The player must destroy it, along with rescuing several miners that barely survived the alien attack. There is also a Sub-Module somewhere, but there is no information from SatScan on its location.
There are two rails on this level, an all weapons power-up module, along with a second one that has blue and green upgrades only, extra life and shields recharge modules.
Enemy forces include ceiling-dripping acid, flying aliens and gun turrets.
There is no information from SatScan on this level (along with the next two levels as well), due to water interference. The player must go underwater through a bit of this level in order to shoot projectiles that will open up an area, along with obtaining tube bombs to blow up a stopper that will drain the water from the level so the player can escape. There is also an anti-pressure module that allows the Attack Ship to dive without using up fuel that must be collected, along with a missile module as well.
Enemy forces include new aliens, several of which are underwater creatures, which also appear on the upcoming two final levels.
Here the player must make their way through a part underwater area, and part cavern area in order to find a mirror laser module, which will power up a device at the end of the level and enable the player to escape. An all weapon power-up, anti-pressure and missiles modules are also available on this mission.
Several of the underwater creatures return from the previous level (which will also appear on the final mission), along with the jumping adversary from level 2, and a new acid meter appears onscreen, which, if the player stays in the liquid area for too long, the acidic levels on their fighter will go up too high and the player will lose that ship. Forcefields also span the entire width of the screen, having buttons on the forcefield modules that must be shot to be deactivated so the player can pass through, along with platforms to navigate around like in Joust.
On this final mission, the player must navigate through this underwater area to find a nuclear crystal, which will enable their ship to not burn any more of its standard fuel. Two keys must also be located in order to open gates so the player can reach the end of the game. An extra life, shield recharge, and all power-up modules are present, and at the very end of the level the player's starting position gets relocated so they will not have to restart at the very beginning of the mission if they die.
Many alien forces return from previous levels, although giant crabs and falling spikes appear for the first time, and the final boss is some entity known as Svin.
- Aside from being another sci-fi shooter that takes place underground, this game doesn't really have anything in common to the earlier, similar-named Atari 2600 game Subterranea, which involved the player shooting a series of bots released by a HEXUPLEX during one stage, then diving underground and destroying a series of targets from there in the second, along with slipping through a series of Electro-Gates.
- If the player is able to beat the game, an ending onscreen message states the player will be sought out if needed again for a similar situation, suggesting a sequel, but no sequel was ever created.