Two campers nearby when a meteor crashes to Earth, and the momentum of the moment shifts from awe to danger in an instant. When they move closer to inspect the strange rock, a lifeform bursts forth from the wreckage and viciously attacks, leaving no doubt that this is a menace from beyond our world.
In a house not far from the crash site, Sam James Brewster and Barb Elissa Neil are preparing to leave town with their two children. Pete [Tom DeFranco], a college student with a keen eye for science, is already tinkering with ideas that might explain more than what their town has seen before. His younger brother Charles [Charles George Hildebrandt], a devoted fan of monster movies, shares Pete’s curiosity but channels it into a lively, sometimes perilous imagination. The family soon finds themselves caught in a night of escalating danger as the meteor’s gift, or curse, begins to reveal itself around them.
As a heavy rainstorm rolls in, Sam ventures downstairs to check the basement for flooding, only to become the first victim of the living horror that has chosen their home as its new lair. Barb follows in an attempt to rescue him, but she too falls prey to the terrifying force. The house becomes a kind of glass box, its walls closing in as the couple’s fate haunts every corner.
Meanwhile, Aunt Millie [Ethel Michelson] and Uncle Herb [John Schmerling] drop by, thinking Sam and Barb have left for their trip. The adults’ presence adds a fragile sense of normalcy to the night, a counterpoint to the creeping dread creeping through the rooms. Pete arranges a study date with classmates Ellen [Jean Tafler], Frankie [Richard Lee Porter], and Kathy [Karen Tighe], hoping to share ideas and perhaps calm the unsettling mood. Uncle Herb, a psychologist preparing for a conference, tries to steer Charles away from the macabre fascination that has captured him, even as he conducts a brief, unsettling session before Charles settles down to sleep in the living room. Aunt Millie then heads out to Bunny’s house [Judith Mayes] for a luncheon with her retired friends, a scene that will soon feel dangerously disconnected from the terror unfolding at home.
In the basement, an electrician confronts a circuit breaker malfunction, and Charles, ever curious, dons a costume and heads down to confront the mystery himself. What he discovers is a hive of slug-like creatures feasting on the remains of the electrician and his mother, guarded by a massive, terrifying queen—the mother monster born from the meteor crash. The eyeless invaders react to sound, a detail Charles quickly learns, and by staying silent he manages to slip away rather than becoming another tastier meal.
Outside and on the move, Ellen and Frankie uncover the dead tadpole-like creature on their journey and piece together that this is unlike anything on Earth. Frankie outfits a possible space origin, while Pete remains stubborn in his skepticism, resistant to ideas that stretch beyond conventional science. Back at Bunny’s, the luncheon guests fall prey to the spawn: the creatures burst forth, overwhelming the adults who fight back only to escape in Millie’s car.
Inside the house, Pete, Ellen, and Frankie seek out Uncle Herb for a professional opinion on the strange visitors, only to discover him being devoured by the relentless spawn. The remaining teens sprint upstairs and barricade themselves in Charles’s bedroom as the adult creature pounds at the door. Charles finds a creative, if dangerous, plan: he activates a radio to lure the monster toward noise and chaos, which in turn triggers an electrical fire that strands the beast. Just in time, Kathy arrives and is pulled into the sanctuary of the room, saving her from a terrible fate.
The group resolves to escape to Pete’s bedroom, where a phone might connect them to help, but the adult creature forces them back into the fray. Pete bolts to another room and climbs onto the roof, while Frankie and Kathy race for the attic and Ellen remains in Charles’s room. The creature tears through the door, seizes Ellen, and tears her head from her body, flinging her remains out the window. Pete returns through the attic window, his mind reeling from the carnage—his parents’ car and the shattered belief that they had left town only deepen his panic. He and Frankie struggle to open the attic door, their fear drawing the creature toward them.
Charles, pressed into action by the escalating peril, concocts a desperate ruse: he rigs a prop head packed with explosive flash powder and a frayed wire for a fuse. He reaches the attic just as the creature seems about to strike, baiting it with the decoy head. The creature devours the prop, momentarily distracting the adult, and a short, brutal tug-of-war ensues as an unfortunate spawn is eaten by the larger threat, buying Charles the chance to reach an outlet. He ignites the fuse, and the ensuing explosion destroys the adult monster, at least for the moment.
With the threat seemingly neutralized, a citywide mobilization begins: policemen and townsfolk comb the area, hunting the remaining spawn and burning the monstrous remnants. Aunt Millie and Charles are backed up by officers as they leave the house, while Frankie and Kathy are seen in the back of a police vehicle, their expressions a mix of relief and trauma. Pete is placed into an ambulance, his mind still fragile from the night’s horrors. That night ends with a quiet, uneasy vigil as a lone patrolman keeps watch on the house, communicating to a neighbor on the CB radio that the spawn has likely been wiped out—until a low rumble is heard. The hill behind the house begins to rise, revealing a colossal, fully grown spawn that promises this is far from over.
