Tom Gregory, a Los Angeles-based sports reporter, is flying into L.A. and lands his private plane after a rough descent through some kind of opaque midair disturbance, only to find the airport deserted. He meets Professor Galvin and his two daughters, Bonnie and Lisa.
The Professor and his daughters tell Tom that the city has been overrun by huge, hulking, slime-covered subterranean beings called Slime People, who appeared out of the sewers and other underground water concentrations. Appearing out of a strange thick fog apparently generated by a device of their own, they've killed hundreds, possibly thousands, panicked the population, fought the army to a standstill, and have now cut off the city with a wall of solidified fog.
The Professor's reasoning that salt would be effective against slug-like creatures gives them a weapon against the Slime People. But freeing the city requires an assault against the creatures' own stronghold, which becomes even more essential when Bonnie is taken prisoner.
- Robert Hutton as Tom Gregory
- Les Tremayne as Norman Tolliver
- Robert Burton as Professor Galbraith
- Susan Hart as Lisa Galbraith
- William Boyce as Cal Johnson
- Judee Morton as Bonnie Galbraith
- The producers originally planned to have creatures called voles in the film as well as the slime people. The voles would have been wolf-like animals from underground that had been domesticated by the slime people, but the were left out of the final script.
- The scenes of devastation supposedly caused by the fight with the slime people (shown briefly while the group is driving away from the airport) is actually footage of the Hollywood Hills shortly after a wildfire swept through the area.
- The scenes in the butcher shop and freezer were filmed in an actual butcher shop owned by the father-in-law of actor/director Robert Hutton in Lancaster, California.
- The two bums who appear in the theater are played by Joseph F. Robertson and Edward Finch Abrams, the film's producer and associate producer, respectively.
- Blair Robertson, who was one of the scriptwriters, plays Mrs. Steel in the TV news footage.
- According to director Robert Hutton in a 1989 interview, designing and making the slime people costumes consumed over half the film's entire budget.
- The only film directed by Robert Hutton, who continued acting in England, where he wrote the script for Persecution (1974), starring Lana Turner (because he didn't like cats).
- In an interview, star/director Robert Hutton said that neither he nor the stuntmen were ever paid for their work in this film.
- Susan Hart didn't have to read for the role of Lisa Galbraith; she was automatically cast on the basis of her attractive looks alone when she showed up for the casting session.
- Because of this film's low budget, Susan Hart was given only $35 to buy her own wardrobe.
- Richard Arlen was originally considered for the role of Prof. Galbraith.