A flying saucer arrives on Earth, searching for planets suitable to raise Gargons, lobster-like but air-breathing monsters that are a reserve food supply on their home planet. Crewman Thor vaporizes a dog named Sparky with a raygun. Another crew member, Derek, discovers an inscription on Sparky’s dog tag and concludes Earth is inhabited by intelligent beings, and therefore they must search elsewhere for Gargon raising grounds, since once full-grown Gargons are a grave menace. The other aliens see no need to spare Earth; they regard themselves as the supreme race, and pride themselves that families and friendships are forbidden on their world. Derek draws a raygun on them and reveals an ancient book distributed by an underground rebellion, which commemorates the more humane periods of their world’s history.