One evening, at a lovers’ lane outside the small town of Dowingtown, teenagers Steve Andrews and Jane Martin see what looks like a shooting star fall to Earth. Hoping to locate the object, Steve and Jane drive through the countryside, while at a nearby shack, an old man finds the mysterious meteorite. After being poked by the old man’s stick, the rock cracks open, revealing a globe of gelatinous substance. The old man picks it up using the stick and is horrified when the clear material suddenly attaches itself to his hand and he cannot get it off. He runs screaming into the road and is almost run over by Steve and Jane, who drive him to the home of Dr. T. Hallen. The doctor, who was about to leave for a medical conference, had already called his neighbor, Mrs. Porter, to bid her farewell, and turned out his lights, but when Steve and Jane pound on his door, admits them. Hallen is baffled by the substance, which now covers the moaning man’s arm and has turned reddish. The doctor asks Steve and Jane to return to the old man’s home to uncover more evidence, while he examines the now-unconscious man. Upon reaching Steve’s car, however, the two teenagers are confronted by braggart Tony Gressette and his pals “Mooch” Miller and Al, who are angry that Steve drove by them so quickly on his way into town. Tony challenges Steve to a drag race, and Steve, adroitly figuring out how to beat Tony, agrees on the condition that they race in reverse. Steve wins the race but is stopped and questioned by tolerant police lieutenant Dave, who lectures Steve but then releases him. Steve persuades Tony and his pals to accompany them to the old man’s cabin, while at the doctor’s office, Hallen, concerned about the spread of the seeming parasite, calls his nurse, Kate, to come in and assist him. In the woods, the teenagers find the empty meteor shell but nothing else, and Tony tries to convince Steve to forget the matter and go to a late-night horror film at the local movie theater. Steve agrees and also acquiesces to Jane’s request to take along the old man’s little dog. While the teenagers are driving back to town, Kate arrives at the doctor’s office and is mystified upon discovering that the old man has disappeared completely. The alien mass has grown considerably in size, and when Hallen sees how large it is, he instructs Kate to throw acid on it, but the liquid has no effect. While Hallen rushes to get his shotgun, the substance engulfs Kate, then goes after the doctor, who has locked himself in his study. Still worried, Steve insists on stopping at Hallen’s office and when there is no answer at the door, walks around the house, where through a window he sees the doctor being devoured by the monster. Terrified, Steve and Jane go to the police station, where Sgt. Jim Bert, who has been the butt of several practical jokes by teenagers, dismisses Steve’s story. Dave insists on investigating, however, and the foursome goes to the doctor’s house. Although there are signs of a struggle in the study, Dave and Bert are convinced by Mrs. Porter that Hallen left town for the conference and decide to wait until the doctor’s estimated time of arrival at his hotel to pursue the matter. While the police are calling the teenagers’ parents, a mechanic is eaten by the ever-growing monster. Steve’s father believes that his son is genuinely disturbed about something, but Jane’s irate father, the high school principal, orders Steve never to see Jane again. Determined to find out what is going on, however, the young couple sneak out of their homes and reaffirm their belief in each other. They then persuade Tony and several others to leave the movies and search for the alien substance. Unknown to Steve and the others, the mass has grown again, devouring several bar patrons. The group splits up and searches the town, with Jane being relieved to discover the little dog near Steve’s father’s grocery store. The couple is upset to find that the store is unlocked, and upon entering, conclude that the monster attacked the night clerk. The alien then comes after them, and with nowhere else to go, Steve takes Jane into the meat freezer. Although the mass squeezes under the door, it quickly retreats, and Steve and Jane are able to escape from the store. Steve tells his friends, and when Bert, still thinking that they are pulling a prank, refuses to believe them, they decide that they must alert the town themselves. The teenagers sound off their car horns, the civil defense siren and even the fire station siren, thereby awakening many people and causing them to assemble at the market. Steve’s earnestness and fear convince Dave that he is telling the truth, but when the scoffing Bert searches the market, they find no evidence of the monster. Just then, the mass attacks the movie theater, and dozens of people are killed, while others run screaming out into the street. Horrified by the carnage, Dave attempts to clear the area and set up defenses. Jane’s young brother Danny, seeing the huge substance coming down the street, shoots at it with his cap gun, and Steve and Jane grab him, taking refuge with him in a nearby diner. The monster then engulfs the diner, trapping Steve, Jane, Danny and the diner’s owner and waitress. Dave concocts a plan to kill the mass by electrocuting it, and although Bert succeeds in shooting a power line, the high voltage has no effect on the alien. Hiding in the diner’s cellar, the owner notices that the power line has started a fire, and when he attempts to put it out with a carbon dioxide fire extinguisher, Steve observes that the encroaching alien retreats from the freezing spray. Realizing that cold stops the monster, Steve yells at Dave to get as many extinguishers as possible. Led by Jane’s father, who breaks into the high school to get twenty extinguishers, the town citizens soon cover the mass with the spray and freeze it. A relieved Steve, Jane and Danny are reunited with their parents, and Dave assures Steve that the military is going to transport the monster to the Arctic, where it will remain frozen and incapable of causing more harm.