The game's plot involves the planet Abadox being attacked in the year 5012 by Parasitis, a techno-organic horror that manages to obliterate nearly the entire defense fleet of Abadox, save for a single ship, piloted by Second Lieutenant Nazal (Nazar in Japan). Nazal goes into Parasitis—not just to continue the attack, but to save Princess Maria who was swallowed whole by it.
The story is a lot different and darker in the Japanese version, likely due to Milton Bradely toning it down for the American market. In the year 5012 of the Essler Calendar, the planetoid-beast Parasitis, thought to have been an unknown person who harbored Parasite X which ultimately turned them into it, is approaching Earth, and no one wants its spawn to rain down on the planet with the kind of death toll that would result. The Earth Allied Forces send a unit of their best men and technology to destroy it...but they're all killed, save for Nazar. As he's mourning them and his lover Maria, a previous victim of Parasitis, he catches a glimpse of the "Mother Alien" at the entity's core. This, somehow, restores his resolve to destroy Parasitis—which feels to him like a fight against the great devil Abaddon...
The game is notable for being really gruesome for NES standards, and it's often compared to Life Force due to its similar organic themes and story (well, the American version as least, the story is very different in the Japanese original, which was called Salamander). Abadox was also notorious for its horrendous difficulty, even for a game of this genre.
This game provides examples of:
- All the Worlds Are a Stage: Downplayed Trope. The final stage has you making a hasty escape through Parasitis's innards, with the maze you must quickly navigate incrementally taking on the appearance of most of the previous areas you fought through.
- Attack Its Weak Point: Most of the bosses have a "sweet spot" that needs to be shot.
- Bittersweet Ending: Nazar beats the Parasitis core, rescues Maria, they make it out and the Eldritch Abomination is annihilated, but so is their home planet. On top of that, Maria may still harbor Parasite X.
- Bowdlerise: Milton Bradley censored references to Abaddon, and instead justified the title (originally a combination of that demon's name and the "Parasite X" that started the whole mess) by making Parasitis's first victim a small planet named Abadox, with Maria the erstwhile Princess of the place, rather than just Nazal's lover. The Abaddon reference makes a lot of sense once you realize Parasitis is demonic in nature, no less—probably something else Milton Bradley didn't want to get into a scandal with. And, of course, the captive Maria was nude in the Japanese version, in which she was clothed in the American version.
- Bragging Rights Reward: Sitting through the credits and starting a new game grants you invincibility. You know, after you've proven that you don't need it (there is a button code to enable it on a first playthrough, though).
- Collapsing Lair: The final stage has no enemies; you must instead navigate your way out of Parasitis before it blows up.
- Digital Kimono: Maria is naked in the Japanese version. This obviously wasn't the case in the American version.
- Eldritch Abomination: Parasitis qualifies as it. Sure, it's primarily a Planet Eater, but speaking biologically, this thing should not be. It doesn't help that in the original Japanese story, the attack on the mother Parasitis is likened to attacking Abaddon, the destroyer of worlds. And the final boss is emblazoned with the picture of a horned humanoid torso. The Japanese story even alleges that Parasitis used to be human. In which case, Nazar and Maria's unborn child was the most likely template.
- Eternal Engine: Stages 5 and 6 are what's left of the hospital vessel the Parasitis originally subsumed. In the Japanese, Parasitis constructed the machinery itself, within itself.
- Evil Is Visceral: The game is full of this trope, being one long Womb Level.
- Evolving Weapon: Collecting power-ups adds upgrades to Nazal's laser gun and rocket launcher.
- Eyes Do Not Belong There: Eyes are everywhere inside Parasitis.
- Faceless Eye: One of the enemies in the first stage, and the miniboss of the penultimate stage.
- Flaming Skulls: An enemy in the first stage.
- Flunky Boss:
- The penultimate boss only attacks if its flanking eyes are present. If the flanking eyes are destroyed, it's a sitting duck.
- The final boss has two minions attack from behind. This angle of approach makes them hard to take out.
- Giant Enemy Crab: The miniboss of stage 4.
- Hard Levels, Easy Bosses: In almost any boss battle, there's one spot on the screen where you can sit and shoot all day without fear of getting hit yourself. The parts leading up to that fight are pure Bullet Hell.
- Gorn: One of the goriest games on the NES. Even the logo is covered in blood!
- Heartbeat Soundtrack: Toward the end of stage 4 as you approach the Cilia Monster.
- Humongous Mecha: The boss of stage 5 is a robot who's about five times the player character's size.
- King Mook: The miniboss of stage 4 is a larger version of the crabs you encounter throughout the level.
- Meaningless Lives: Losing all your lives only results in your score being reset. You don't even lose your checkpoint. However, since this means you'll have to go through the second half of a stage with no power-ups, this adds to the game's difficulty.
- Mirror Boss: The miniboss of stage 5 consists of three robots who are about the same size as the player character and use the same weapon.
- Nintendo Hard: The last few stages dip into Bullet Hell territory without the tiny hitbox that makes most Bullet Hell games easier than they look.
- One Bullet at a Time: All the player's weapons are limited to either one or two shots or a single "spread" on the screen at a time.
- Planet Eater: Parasitis in the Milton Bradley version. It's more of a Generic Doomsday Villain in the Japanese, although potentially also the Antichrist.
- Rings of Death: A weapon not available until stage 3 fires giant rings.
- Save the Princess: Only in the American version. This was absent in the Japanese version as Maria is not the princess, but Nazal's lover. She's even Floating in a Bubble!
- Smashing Hallway Traps of Doom: Appears in the first part of stage 6. The single-piece crusher is almost as tall as what can be shown on screen.
- Trial-and-Error Gameplay: Without sheer luck, there is no way you will dodge the hands in stage 2 or the phoenixes in stage 6 the first time you play.
- Unintentionally Unwinnable: If the player is using the in-game invincibility cheat, and is latched onto by one of the grasping creatures that can attach to your ship in the second and third to last level, it becomes possible to get stuck at the boss fight. The player cannot position themselves high enough on the screen to shoot the boss, and cannot die or get rid of the creature limiting their movement.
- Womb Level: You're inside a Planet Eater for most of the game.