TEENAGERS FROM OUTER SPACE

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.

The Captain and Thor disarm Derek. Taking him prisoner, they plan to have Derek tried and executed. The Gargon they brought with them falls sick to Earth's atmosphere, indicating that the planet is unusable. While his crew members are distracted, Derek escapes on foot. The Gargon regains consciousness, having adapted to the atmosphere. When the Captain gives his report, he is informed that Derek is the son of the Leader of their race, although he is unaware of this. Thor is sent to hunt down Derek, with orders to bring him back alive or kill him and any other intelligent beings to protect their mission to Earth. The rest of the crew returns to their home world, leaving the Gargon in a nearby cave.

Derek finds the address on the dog's tag, where he meets Betty Morgan and her Grandpa. They have a room to rent, and Derek inadvertently becomes a boarder. When Betty's friend, reporter Joe Rogers, cannot make it to their afternoon swim at Alice Woodward's place, Derek tags along with Betty. He shows the tag to Betty, who recognizes it. Derek takes her to the place where the spacecraft landed. He shows her Sparky's remains and describes Thor's weapon, that can also vaporize humans. Betty vows to help Derek stop his crew mate.

Betty and Derek have several run-ins with Thor, who vaporizes several humans (including Alice and Betty's teacher, Professor Simpson) to keep them from raising the alarm. Joe follows up on stories of skeletons popping up all over town. Thor is wounded in a shootout with the police, and kidnaps both Derek and Betty to help him receive medical attention, in the process revealing Derek's parentage to them. Two car chases and a gunfight follow. Thor is captured by Earth authorities after plummeting off a cliff in a stolen car.

The Gargon grows immensely large after devouring a policeman investigating the aliens' landing site and attacks numerous people. Derek and Betty go to the car wreck site to look for Thor's raygun. They kiss and Derek vows, "I shall make the Earth my home. And I shall never, never leave it." The Gargon appears. Derek finds the raygun under a rock, but it is damaged and unable to fire. The Gargon heads towards the town. Derek and Betty follow and confront it, using electricity from the overhead power lines to fuel the raygun's components and kill the Gargon. The invading fleet appears in Earth's orbit.

Derek retrieves Thor from the police and forces Joe to drive him to the landing site, using the damaged raygun as a bluff. Betty is baffled as to why Derek is acting so brutish and seemingly going back on his vow, but she heeds his plea for her to trust him and does not tell Joe that the raygun is inoperative. At the landing site Derek reunites with the Captain and meets his father for the first time. Derek pretends to feel regret for his insubordination and offers to guide the spaceships to land. Derek then goes into the spacecraft, closes the hatch to cut off the other aliens, and leads the invasion fleet at full speed directly towards his ground location, destroying them all. Derek's father, the Captain and Thor are killed in the process. Betty realizes that Derek in fact kept his vow.

Derek and Betty confront a giant Gargon after a series of pursuits; Derek chooses to stay on Earth, sealing his bond with Betty and facing the alien threat.

The ending emphasizes the triumph of compassion and connection over alien indifference. Derek’s alliance with Betty represents hope for coexistence and resistance against destructive conformity. The defeat of the Gargon and Derek’s decision to remain highlight the film’s campy yet sincere message: sometimes love (and a handy raygun) can save the world.