When a giant asteroid dubbed "Flora" threatens the Earth, a mission to destroy it is quickly launched, led by hard-ass commander Rankin (Robert Horton). The disaster is successfully averted; however, a small piece of space slime is unwittingly smuggled back to the space station Gamma 3, where it begins feeding off the station's power cables and grows into a tall green monster with deadly, electrified tentacles. Worse, the creature heals rapidly and regenerates any severed anatomy into a whole new creature... eventually building up a veritable army of nigh-unstoppable monsters. Now Rankin and the Gamma 3's crew must fight a seemingly hopeless battle as well as interpersonal tensions in an effort to keep the beasts from reaching Earth.
This film is notable for being the first movie ever riffed by Mystery Science Theater 3000, on the pilot episode for the original KTMA-era show. Sadly KTMA lost the rights to the film shortly after the episode was made, making it a Lost Episode (and, given Best Brains' attitude toward their KTMA-era work, it's likely to remain lost. Keep Circulating the Tapes!).
The Green Slime contains the following tropes:
- Artistic License Military: "Captain" Martin is outranked by "Commanders" Rankin and Elliot. One might presume that in this case, "Commander" is a title rather than an actual rank.
- Asteroids Monster/From a Single Cell/Healing Factor: What makes the slime creatures so dangerous (besides the fact that they, you know, kill people), and the reason they must be kept away from Earth at all costs.
- Closed Circle: Due to the Green Slime's From a Single Cell nature, Gamma 3 must go into a self-imposed quarantine to prevent them from wiping out Earth's population. For bonus points, the station is actually circle-shaped.
- Colony Drop: The whole plot starts when the Rogue Planet "Flora" wanders into the Solar System and threatens to crash into Earth.
- Commander Contrarian: Cmdr. Elliott constantly makes decisions that end up endangering the station further just because he doesn't like being told what to do by Rankin.
- On the other hand, Rankin constantly emasculates Elliott in front of the crew, up to and including trying to take Lisa back because he can't accept she loves Elliott.
- Cyclops: The slime creatures have a single, huge CBS-like eye.
- Red Eye Take Warning: The slime creatures' one eye is red, and they are very dangerous to human life.
- Energy Absorption: The slime creatures can seemingly feed off of any type of energy, including sunlight and even Frickin' Laser Beams. On the other hand, they burn up just fine.
- For Want of a Nail / Nice Job Breaking It, Hero: On the Flora asteroid, Rankin angrily throws away the scientists' collection device, causing it to shatter and spattering a crewman's spacesuit with the slime, which brings it onto the station and causes the disaster. If he'd just have put it down carefully, the carnage would've been avoided.
- Green Is Gross: The titular slime. It's a glop that drips and oozes around, and as soon as it hits electrical power it turns into a Green and Mean tentacle monster.
- Halfway Plot Switch: The plot with Rogue Planet Flora takes up only the first act, and is resolved rather quickly. It exists only to explain where the slime creatures come from and how they get onto the space station.
- Heroic Sacrifice: Elliott finally does one thing right by Taking the Bullet for Rankin when the two of them set Gamma-3 to burn up in re-entry.
- Idiot Ball: Lisa, the station's chief medical officer, violates quarantine procedure when the mission returns from Flora by running in to greet them in the landing bay before they've gone through decontamination.
- Impaled with Extreme Prejudice: Because Throwing Your Rifle Always Works.
- Jerkass: Both Rankin and Elliott qualify, as they continue their personal bickering even during the station-threatening emergency.
- Love Triangle: Cmdr. Elliott and Lisa are engaged, but Lisa was previously in a relationship with Cmdr. Rankin as well.
- The Medic: Lisa is the chief nurse in Gamma-3 and eventually the main doctor when Halverson's stupidity gets him killed.
- Monochrome Casting: Despite the space station supposedly having an international crew, everyone is Caucasian. Pretty odd for a film made in Japan, even if for American audiences.
- Noodle Incident: We never get too many details of the incident that rent Elliott's and Rankin's friendship. We do know that Elliott had mounted an attempt to save one life and ended up costing ten, and that Rankin duly reported this.
- Raygun Gothic: All over the place. The ships all look like Retro Rockets, and even if Gamma-3 is donut-shaped, the Artificial Gravity is set so it pulls toward the station's "bottom" rather than its outer circumference.
- Red Shirt: Michaels, Halverson's assistant, is the first to go.
- Rogue Planet: The whole plot starts with the planet Flora wandering into the Solar System on a collision course with Earth.
- Shock and Awe: The creatures' tentacles are so charged with energy, a mere touch is deadly.
- Suspiciously Specific Denial: When Rankin first arrives on the space station, the very first thing Lisa says to Elliott is that she no longer has feelings for Rankin.
- Throwing Your Gun at the Enemy: During the climax, Rankin's laser gun runs out of power, so he throws the useless weapon at the attacking creatures.
- Too Dumb to Live: Dr. Halversen runs into a part of the station which is currently overrun by slime creatures in an attempt to retrieve copies of his work. Needless to say, it doesn't end well for him.