It follows the story of a crash-landed alien that finds refuge in the basement of a house and grows to monstrous proportions, eating those unlucky enough to venture down. A handful of teenagers try to survive the onslaught of the creature and its young.
Two campers are nearby when a meteor falls to Earth. When they go to investigate, they are attacked and eaten by a bizarre life form that emerges from the crashed rock.
Near the crash site is a large house. The house is the home of Sam (James Brewster) and Barb (Elissa Neil), and their two children, college student 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 early to go into town, which Barb hopes can be put off because of the incessant rainstorm. Sam insists, and he goes downstairs to check the basement for flooding. Down there, he is eaten by the bizarre monstrosity. Barb suffers the same fate when she goes looking for him.
Aunt Millie is awakened by their screaming, but she attributes it to the horror movie that Charles is watching on television in his room. Charles, a monster movie fan, tries to scare his aunt with a monster costume and some flash powder. Pete, a budding scientist, sets up a study date with classmates Ellen (Jean Tafler), Frankie (Richard Lee Porter), and Kathy (Karen Tighe). Uncle Herb, a psychologist, wants to investigate Charles's interest in the macabre, and he holds a brief interview with the boy then falls asleep in the living room.
Aunt Millie heads over to her mother Bunny's (Judith Mayes) house for a luncheon with her retired friends. When an electrician arrives to investigate a circuit breaker malfunction in the basement, Charles dons a costume and goes down to scare him. There, he discovers the basement is swarming with slug-like creatures. Charles follows them until he finds them feasting on the electrician's body. Around are some larger versions of the creatures: a huge one, presumably the original, has developed three different heads. The smaller ones seem to be spawn given off by the adult monster. Realizing that the eyeless creatures react to sound, he stands silently while the large creature disgorges his mother's head for the spawn to feast on.
Meanwhile, Ellen and Frankie have discovered one of the tadpole creatures dead on the way over to the house, and deem it unlike any animal on Earth when they dissect it. Science-fiction fan Frankie hypothesizes that the creature could be from outer space, but hard-nosed scientist Pete dismisses that theory. At Bunny's house, Millie arrives and they prepare the luncheon, unaware that the spawn have infested the house. One of them gets into her food processor, tainting the vegetable sauce she is making. When her guests arrive, the arm-sized creatures suddenly emerge and attack them. The women fight back and manage to escape in Millie's car.
Back at the house, Pete, Ellen and Frankie seek out Uncle Herb to get his opinion on the creatures but find that he has been killed and mutilated by the spawn. The three-headed adult creature charges them, and they run upstairs to barricade themselves in Charles's bedroom. Charles distracts the adult by turning on a radio, which it eats, causing an electrical fire which burns it. Pete and the others then see Kathy arriving, and their warnings to her go unheard over the din of the rain. The teens pull her into the bedroom just in time to save her from the beast.
The teens decide to head for Pete's bedroom, where there is a phone to call for help with. However, the adult creature ambushes them. Pete runs to another room, Frankie and Kathy run up to the attic, while Ellen stays in Charles' bedroom. The creature easily breaks down the bedroom door, then bites her head off and defenestrates her body. Pete climbs out onto the roof and sees her body on the ground.
He also notices his parents' car, still in the garage, and realizes they never left and must have died. Kathy and Frankie see the body as well, and Pete crawls in through the attic window. Traumatized by Ellen's death, he becomes unhinged, fighting with Frankie to open the attic door, and the shouting noises attract the creature. Meanwhile, Charles has concocted a plan based on his success at burning the monster earlier: he has filled a prop head with explosive flash powder, with a frayed electrical cord trailing behind to act as a fuse. He attaches the head to a ski pole and holds it up in front of the creature's mouth.
The creature notices Charles and leaves Kathy and Frankie alone. When Charles screams, the noise spurs the creature to devour the prop head. However, the cord proves too short to plug into an outlet. One of the arm-sized spawn creatures appears and bites into Charles's shoulder. Fortunately, the smaller creature gets in the way of the adult when it lunges at Charles, and it ends up chewing on it instead. Now that the mouth is close enough, Charles manages to get to the outlet, igniting the powder and blowing up the adult.
With the threat no longer a secret, a massive hunt for the spawn has been mobilized. Policemen and townspeople go around killing spawn and burning the remains. Aunt Millie returns to the house to care for Pete and Charles as best she can, while Frankie and Kathy are taken away in an ambulance. That night, a lone patrolman stands guard outside the house. His contact on the CB radio is confident that the spawn has been wiped out, but then the patrolman hears a low rumbling, and sees the hill by the house lift up, revealing that it sits atop the mouth of a colossal spawn.
Producer Ted Bohus said that he conceived the idea for The Deadly Spawn in 1979, and that he was inspired by an article in National Geographic about seed pods that were recovered from the Arctic. According to Bohus, he created an initial creature design that involved a man in a suit, but associate producer and effects director John Dods was unenthusiastic about that prospect. Several days later, Dods returned with several alternatives, including the "Mother Spawn" that was eventually used in the film.
Actor and director Tim Sullivan got his start in film as a 15-year-old production assistant on The Deadly Spawn. Dods was the brother of Sullivan's art teacher, and Sullivan earned the chance to work on the film as a result of that relationship. Among other tasks, Sullivan assisted in the manipulation of the main spawn puppet, which was made of rubber and controlled from below by wires.
It has been suggested that the character of Ellen, Pete's girlfriend, was killed off because Tafler got another acting job, but screenwriter McKeown disputed this. He decided early on to shatter the conventional expectations of the audience and maybe the rules of the genre, in order to justify Pete's subsequent breakdown. The horrific effect on the audience of such randomness was to be a bonus. In any case, Ellen's fate was a spectacular shock, and Kathy, the "second lead," wound up being the one who survived.
Much of the film was shot in Gladstone, New Jersey; New Brunswick; and the Palisades Park home of the producer.
The film score by Michael Perilstein was released by Perseverance Records on December 21, 2004. AllMusic awarded it 3.5 out of 5, with reviewer Jason Ankeny describing it as an "innovative score" that "deserves greater notoriety". Ankeny praised its atmosphere, and said that it successfully reached a "balance between serious musical aspirations and the tongue-in-cheek demands of the material".
AllMovie gave the film a positive review, describing the film as an "engaging, exciting sci-fi/horror adventure with realistic characters, effective acting and a willingness to betray expectations." The website Cool Ass Cinema wrote that the film is "not out to win any awards, it's simply there to entertain and in the most energetically gruesome fashion possible". James Rolfe of Cinemassacre.com said the film "has some horrendous acting and very dull moments, but there's enough blood and guts to keep any gorehound satisfied. And for the small budget they had, they did an unbelievable job".