An alien spacecraft comes to Earth, while searching the galaxy for a planet suitable to raise "gargons," a lobster-like (but air-breathing) creature which is a delicacy on their homeworld. Thor (Bryan Grant), the lead alien, shows his contempt for Earth's creatures by vaporizing a dog named Sparky. Crewmember Derek (David Love), after discovering an inscription on Sparky's dog tag, fears that the gargon might destroy Earth's local inhabitants, making the other spacemen scoff. Being members of the "supreme race", they disdain "foreign beings," no matter how intelligent and pride themselves that families and friendships are forbidden on their world. Derek turns out to be a member of an underground which commemorates more humane periods of his world's history. Their one gargon seems to be sick in Earth's atmosphere. While his crewmates are distracted, Derek flees. Eventually, the gargon seems to revive. When the Captain reports Derek's actions, he is connected to the Leader (Gene Sterling) himself. It turns out that Derek is the Leader's son, though Derek is unaware of this. Thor is sent to hunt Derek down, with orders to kill to protect the mission. They return to their base, leaving the gargon behind.

Meanwhile, Derek finds the home address found on the dog tag. He meets Betty Morgan (Dawn Anderson) and her Grandpa Joe (Harvey B. Dunn). They have a room to rent, and Derek inadvertently becomes a boarder. When Betty's boyfriend, reporter Joe Rogers (Tom Graeff), can't make their afternoon plans, Derek tags along with Betty. He shows the tag to Betty, who recognizes it immediately. Derek takes her to the place where the ship landed, and shows her Sparky's remains. She doesn't believe him, so he describes Thor's weapon that that vaporizes flesh. Betty takes this surprisingly well and vows to help Derek stop the bad guys. For the rest of the day Betty and Derek have several run-ins with Thor and Joe follows up on stories of skeletons popping up all over town, including in Alice's swimming pool. Eventually Thor is wounded and he kidnaps Betty and Derek for medical attention, he reveals Derek's true parentage. Two car chases and a gunfight ensue and Thor is finally captured by authorities after plummeting off a cliff in a stolen car.

But there are bigger problems: the gargon has grown immensely, killing a policeman investigating the landing site and attacking numerous people. Derek and Betty go to the car wreck to look for Thor's gun. They share a kiss, and Derek vows to stay on Earth. When the gargon ruins their romantic moment, Derek finds the gun just in time for them to escape. Unfortunately the gun is damaged and the monster is heading toward the town. They head out once again to confront it, using power lines to fuel the disintegrator.