Alien invaders pose as rocks in a geologist's lab and discuss their plans for invasion. However, Paul Cameron (Robert Culp) is able to hear them thanks to a metal plate engraved into his skull.
- Alien Invasion: These particular aliens are invading one human at a time.
- Aliens Speaking English: In this case it's aliens thinking telepathically in English.
- Big Creepy-Crawlies: The aliens transform into half-spider/half-crab parasites when they attack humans.
- Bittersweet Ending: As so often happens in The Outer Limits, the world is saved at a terrible price.
- Death of Personality: The aliens' victims suffer this; as the parasites take over their minds and bodies, the people they once were are destroyed.
- Driven to Suicide: The aliens try to do this to Paul, but it doesn't work.
- The Film of the Book: The episode is based on a novel by Louis Charbonneau, but takes major liberties with the original story.
- Happily Married: Paul and Laurie, until the parasites enter their lives (and Laura's body).
- Properly Paranoid: One of the major themes of the episode. Neither Paul nor the people around him are sure of his sanity, but the aliens really are out to get him.
- Puppeteer Parasite: The aliens, although the people they possess are so severely transformed that they become Parasite Zombies who are unable to pass as normal.
- Screaming Woman: Laurie when the parasite attacks her — and who can blame her?
- Telepathic Spacemen: Played with. The aliens communicate with each other telepathically, but their mental contact with Paul is unintentional.