Story: (from http://en.wikipedia.org/wiki/Teenagers_from_Outer_Space)
A team of spacemen arrive on Earth in a space ship. They have been searching the galaxy for a planet suitable to raise their herd of "gargons", a lobster-like (but air-breathing) creature which is a food staple on their homeworld. Thor (Bryan Grant), the first spaceman to emerge, shows his contempt for other-worldly creatures by using his vaporization weapon on a dog, Sparky. Derek (David Love), upon discovering an inscription upon Sparky's dog tag, voices his fear that the herd of gargon might destroy Earth's local inhabitants. The other spacemen scoff. As members of the "supreme race", they are disdainful of "foreign beings", no matter how intelligent, and pride themselves on the fact that families and friendships are forbidden on their world. Derek reveals himself to be a member of an underground which commemorates earlier, more humane periods of the homeworld's history.
The gargon seems unable to thrive in Earth's atmosphere. During this distraction, Derek flees, before the gargon revives and flourishes. When the Captain reports Derek's antics, he is connected to the Leader (Gene Sterling) himself. It turns out that Derek is the son of their Leader, though Derek is not aware of this. Thor is sent to retrieve Derek, with orders to kill if necessary to protect the mission. They return to their base, leaving the gargon behind, hoping to judge its growth rate by the time they return.
Meanwhile, Derek follows the address on the dog tag to a private home that belongs to 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 goes along to visit Alice (Sonia Torgeson). 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 "focusing disintegrator ray" that strips flesh from bone. Betty takes this well, and vows to help Derek stop the bad guys.
During the course of the day Betty and Derek have more than one run in with Thor and his ray gun, and Joe grapples with news stories of skeletons popping up all over town, including in Alice's swimming pool. Eventually Thor is wounded, and when he kidnaps Betty and Derek in order to seek 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 than Thor: the gargon has grown immensely, killing a policeman with whom Joe was investigating the landing site, and attacking numerous people. Derek and Betty immediately head out to the car wreck to look for the gun. Derek and Betty share a kiss, and he vows to make Earth his home and never leave. When the gargon ruins their romantic moment, Derek finds the gun just in time for them to escape. Unfortunately the gun has been damaged in the crash, and the monster is coming towards the town. They head out once again to confront it, using power lines to fuel the disintegrator. They kill the gargon, but it's too late: enemy ships have appeared overhead.
The whole gang, including Joe and Grandpa, hurry out to the landing site. Derek reunites with his father, and makes the ultimate sacrifice to save the Earth by leading the fleet directly into the hillside and causing a massive explosion. Derek does not survive, but is remembered to have said that he would "make the Earth my home, forever".