Two campers are nearby when a meteor falls to Earth and lands in the woods near their campsite. When they go to investigate, they are attacked and eaten by a bizarre lifeform that emerges from the crashed rock, mostly unseen but apparently consisting of multiple heads with hundreds of teeth.
Nearby the crash site is a large house located on a rural plot of land. The house is the home of Sam (James L. Brewster
) and Barb (Elissa Neil
), and their two school-age children, high school age Pete (Tom DeFranco
) and his younger brother Charles (Charles George Hildebrandt
). Visiting are Aunt Millie (Ethel Michelson
) and Uncle Herb (John Schmerling
Sam and Barb rise before anybody else is awake; they have an unspecified trip planned, which Barb hopes can be put off because of the incessant rainstorm. Sam insists, and he goes downstairs to check the basement for flooding. A flapping basement window suggests that the creature from the meteor has taken refuge in the cellar, and sure enough, when Sam goes into the basement, he is eaten by the bizarre monstrosity. Barb suffers the same fate when she goes down to see what happened to Sam; the creature corners her and viciously bites off her face.
Aunt Millie is awakened by screaming, but she attributes it to the horror movie that Charles is watching on TV in his room. Charles is obsessed with magic, monsters, and scaring people. When Aunt Millie returns to call Charles to breakfast, he surprises her by wearing a scary mask and costume and setting off flash powder when she opens the door. Pete disapproves of Charles' hobby, being more interested in schoolwork and science. Pete fancies himself a scientist, and he sets up a study date with three classmates, Ellen (Jean Tafler
), Frankie (Richard Lee Porter
), and Kathy (Karen Tighe
Aunt Millie has plans to have lunch with her mother, Bunny (Judith Mayes
), who is planning an extravagant vegetarian lunch for a small group of friends. Uncle Herb, who is a psychologist, is interested in talking with Charles about his interest in the macabre, and Pete suggests that Herb has been asked by their parents to analyze his brother. Herb asks Charles how real the monsters are to him, and if he ever thinks he'll see one in real life. Charles seems to be well grounded, however, understanding that monsters are not real.
Uncle Herb falls asleep in the living room, and Aunt Millie heads over to her mother's house to help set up the luncheon. An electrician arrives to investigate a problem with the circuit breaker box in the basement of the house, and Charles sees an opportunity to scare somebody. Slipping on his mask, he creeps down into the basement, which is flooded with several inches of water. But he discovers something strange; the basement is swarming with small tadpole-like creatures, the size of small fish. Charles follows them silently until he finds the electrician's body in a corner, with hundreds of the smaller creatures feasting on it. The creatures are all of varying sizes, from the small fish-sized tadpoles, to arm-sized worms, and there is a huge one, presumably the original, that has developed three different heads. The smaller ones seem to be spawn given off by the adult monster, each with the potential to grow into a full sized threat. The creatures have no eyes, and Charles discovers they are sensitive to sound in which their open mouths, filled with hundreds of sharp teeth, seem to be their receiver of sounds since they apparently do not have ears either. The large three-headed creature almost eats him when he snaps his fingers, and he is only able to distract it by tossing his flashlight into a far corner and allowing it to go after the noise. Adding to the horror, he sees Barb's disembodied head lying on the cement floor, partially consumed by spawn, and realizes that the creatures have killed his parents. Charles stands motionless against a basement wall, unable to move in fear of being discovered by the alien creatures.
Meanwhile, Ellen and Frankie arrive for the study session in a flurry of excitement; they have discovered one of the creatures dead on the way over to the house, and they've brought along its body to dissect in an attempt to understand what it is. Frankie hypothesizes that the creature could be from outer space, but Pete is hostile to that idea and dismisses it as ignorance. Ellen reminds Pete that to make suppositions, scientists must have some imagination.
At Bunny's house, Millie arrives and helps her set up for her guests; Bunny is unaware that the spawn have reached the house and are lurking in her kitchen. One of them gets into her food processor, tainting the vegetable sauce she is making. When her guests arrive, all of them older ladies, they are horrified when they taste the sauce with the dead spawn in it, and the arm-sized creatures suddenly come at them from all angles, biting at fingers and toes, latching onto their legs and arms like small leeches. The women fight back and smash and kill a number of the creatures, all of them bearly escaping with their lives in Millie's car.
Back at the house, Pete, Ellen and Frankie seek out Uncle Herb to get his opinion on the strange creature they've found. They find Herb's body in the living room, with hundreds of spawn eating him from the inside out. The three-headed adult creature confronts them in the hallway, and they run upstairs to Charles's bedroom. Charles, who understands that the creature will chase after them because of the noise they are making, distracts it with a radio, which it eats. Before the creature can find Charles and kill him, the doorbell rings--Kathy has finally arrived at the house. Pete and the others try to warn her from an upstairs window, but she doesn't hear them and goes into the house anyway. The monster nearly grabs her in the hallway outside Charles' bedroom, but she escapes its huge mouth.
Desperate to summon help, the teens devise a plan: they will make a dash for Pete's bedroom, where there is a telephone, and they'll call for help. But the monster ambushes them in the hallway. Pete runs to another room, Frankie and Kathy run upstairs to the attic, while Ellen stays in Charles' bedroom and slams the door. The creature, attracted by her shouting, knocks the door down and rushes at her. Before Ellen can escape out the window, the monster bites her head off and her headless body falls out into the front yard of the house.
Pete climbs out onto the roof and sees Ellen's body on the ground below. He also notices his parents car, still in the garage, meaning they never left. Kathy and Frankie see Ellen from the attic window, too, and Pete eventually climbs in through the small opening. He is in shock and is delirious, telling them "Ellen's really scared." When he tries to open the barricaded door, Frankie tries to stop him but Pete becomes unhinged, fighting with Frankie and screaming for Ellen. Frankie knocks him unconscious, but it's too late, his shouting attracts the creature. Fortunately, Charles has concocted a plan. Rushing to his bedroom, he assembles a hollow head full of flash powder, embedding a frayed electrical cord in the explosive material.
As the creature advances on Frankie and Kathy, Kathy is hysterical and screaming. Charles rushes up the stairs and tells them to be quiet, then he baits the creature with the fake head. At first it refuses to eat it, until Charles gets the idea to scream. The monster immediately devours the head; with the dangling power cord hanging just outside its mouth. Using extension cords, Charles lures the creature closer and closer until he is close enough to an electrical outlet to plug it in. However, one of the arm-sized spawn creatures lands on his right shoulder from the above rafter and begins bitting into him. Charles is forced to use a screwdriver to stab at the spawn attached to his shoulder until the three-headed mother spawn approaches and inadverantly eats the wounded spawn creature from Charles' shoulder, thus freeing him. Charles manages to get at the extension cord and plugs it into an outlet, and the three-headed spawn creature instantly explodes in a gory shower of alien blood and body parts alll over the attic interior.
The threat no longer a secret, a massive hunt for the spawn has been mobilized at the house and the surrounding areas as dozens of townspeople converge on the area. Aunt Millie arrives back at the house and cares for the wounded and shaken Pete and Charles as best she can, while policemen and volunteers scour the surrounding woods and bushes for spawn, which they stun with cattle prods or stab with shovels, picks and axes. The creatures are dumped onto various bonfires that the locals set, and are disposed of, including the remains of the three-headed eight-foot mother spawn. Frankie and Kathy are also shaken but alive, and are taken away in an ambulance.
In the final scene, several hours later, night falls. The house, now deserted except for a posted patrolman, seems to have been cleared of all the aliens--that is, until the patrolman hears a rumbling in the ground. It seems as if an earthquake is hitting, until the entire hillside next to the house erupts with the head of a gigantic spawn, even larger than the house itself. Looks like somebody missed one!