The film concerns a race of subterranean reptile-men (dubbed "slime people", due to their slime-covered skin) who create a wall of "solidified fog" around Los Angeles and proceed to invade the city. A pilot (portrayed by Hutton) lands in Los Angeles after some flight difficulties, and finds the city almost deserted. He later encounters other survivors, including a Marine separated from his unit, and a scientist and his two daughters, and the group does their best to halt the further invasion of the slime people.