A flying saucer arrives on Earth while searching for planets suitable to raise "Gargons", a lobster-like but air-breathing monster that is a delicacy on their home planet. Crewman Thor shows his alien contempt for Earth's creatures, by needlessly vaporizing a dog named Sparky with a disintegrator raygun. Another crew member by the name of Derek, discovers an inscription on Sparky's dog tag and fears the Gargons might destroy Earth's native inhabitants. This makes the other aliens scoff at the thought. Being members of the "supreme race", they disdain "foreign beings", no matter how intelligent; they pride themselves that "families" and "friendships" are forbidden on their world. Derek reveals an ancient book and turns out to be a member of an underground rebellion, that commemorates the more humane periods of their world's history.
The Captain tricks him and Thor disarms Derek. Taking him as prisoner, they plan to put Derek on trial and have him executed by the high court. The Gargon they brought with them suddenly falls sick to Earth's atmosphere. While his crew members are distracted, Derek escapes on foot. Eventually, the Gargon revives from being unconscious. When the Captain gives his report, it is revealed 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 return to their homeworld, leaving the Gargon behind in a nearby cave.
Meanwhile, Derek arrives at the address he found 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 immediately. Derek takes her to the place where the spacecraft landed and shows her Sparky's remains. She does not believe him, so he describes Thor's weapon that can also vaporize humans. Betty takes this surprisingly well and vows to help Derek stop his crewmate.
For the rest of the day Betty and Derek have several run-ins with Thor, who vaporizes several humans, and Joe follows up on stories of skeletons popping up all over town. Eventually, Thor is wounded, after a shoot out with the police. And he kidnaps both Derek and Betty to help him receive medical attention, in the process revealing Derek's true parentage to them. Two car chases and a gunfight follow, and Thor is finally captured by Earth authorities after plummeting off a cliff in a stolen car.
Shortly after, the Gargon grows immensely large, after killing and devouring a policeman investigating the alien's landing site, and attacking numerous people. Derek and Betty go to the car wreck site to look for Thor's raygun. They share a kiss, and Derek vows to stay on Earth. The Gargon suddenly appears and ruins their romantic moment, but Derek finds the raygun under a rock just in time for them to escape. Unfortunately, it is damaged and out of power. The giant Gargon begins heading towards the town. They follow and confront it, having used the electricity from the overhead power lines to fuel the raygun's components. Derek eventually kills the monster, but it's too late. The invading fleet appears in Earth's orbit.
Derek retrieves Thor from the police and everyone, including Joe and Grandpa, hurry to the landing site. He then reunites with the Captain and meets his father for the first time. Derek pretends to feel regret for his insubordination and offers to help guide the spaceships to land. Derek then goes into the spacecraft alone and makes the ultimate sacrifice, leading the invasion fleet at full speed directly towards his ground location and causing a massive explosion, killing his father, the Captain and Thor in the process. Derek does not survive the blast but is remembered by Betty for declaring, "I shall make the Earth my home. And I shall never, never leave it."
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