In Riverdale, Illinois, a man carrying a lighted glass container collides with a pedestrian; the container shatters and a violent struggle ensues, setting a tense, mysterious tone that threads through the town’s quiet surfaces.
Glenn Cameron [Alan Jay Factor] and his fiancée, Elaine Cameron [Jody Fair], are headed home when a strange light draws their attention into nearby woods. They pause to investigate and uncover a charred scene of dead animals, leading them to an imposing metal structure that resembles a rocket’s nose cone and hints at something far beyond ordinary science or superstition.
Two days later in Washington, D.C., a government committee reviews classified army footage of the object. Sen. Walter K. Powers [Cornelius Keefe] and his assistant Dan Walker [Robert Ball] fly to Riverdale to inspect the situation and meet Glenn Cameron [Alan Jay Factor], whose father, the mayor, is missing. The trio is joined by Alice Summers [Joanna Lee], the mayor’s secretary, who helps Dr. Paul Kettering [Ed Nelson], the committee’s lead investigator, by recording their test results. The senator approaches the cone to question Kettering and his assistant, Dr. Wyler [David Hughes], who confirm the object’s apparent indestructibility and reveal its interior as a maze of tunnels.
Back in Riverdale, Mayor Cameron returns to his office behaving as if possessed. He pulls a pistol and struggles to aim it at his head as Kettering, the senator, Alice, and Glenn arrive. Kettering notices a strange mound near the mayor’s neck and probes for answers. The mayor lashes out, and chaos erupts until a deputy shoots and kills him in the hallway. During the autopsy, the medical team discovers a dead creature attached to the mayor’s neck; it has injected a toxin into his nervous system, ensuring death within 24–48 hours even if he had not been shot.
As the investigation widens, a sheriff encounters a man with a glass container on a road, and a struggle ensues in which the sheriff is knocked unconscious. The other man with a container watches as two additional figures remove something from it. The sheriff revives and the trio drives away, leaving more questions than answers in their wake.
With Alice’s help, Kettering experiments on a piece of the mayor’s parasite. The parasite behaves like a living attachment, and Kettering frees himself by burning it away. Wyler calls Kettering, and they set out toward the cone again. En route, they discover an abandoned electric utility truck and learn that a wider threat is unfolding. Powers tries to warn the governor, but the telegraph lines become compromised as the town struggles to respond.
Three search parties fan out: Kettering and Alice in one direction, Glenn and Elaine in another, and the sheriff’s team in a third. They uncover two glowing containers holding more parasites. When the telegraph office becomes a bottleneck, the lines stay busy or fail, complicating a coordinated response.
Three men descend on Alice’s apartment and plant a parasite in her room; she is soon overtaken and joins their scheme. Realizing something is terribly wrong, Paul and Glenn rush back toward the cone and encounter Prof. Helsingman [Saul Bronson], a dying scientist who had vanished five years earlier with a missing expedition. Helsingman’s neck bears marks similar to the mayor’s, and he cryptically utters the clue that points toward a broader reach. He is taken to the hospital, where his condition is confirmed, and the team begins to piece together a larger, more frightening pattern.
Powers attempts to contact allies, but the lines are busy or compromised, stalling critical warnings. Glenn and Paul hurry to the telegraph office to verify whether the warning reached the governor, and they are attacked but manage to subdue their assailants and escape.
Kettering climbs the cone to inspect his equipment and realizes the two deputies guarding the entrance are now possessed; they are shot, and the pair collapses inward. With Glenn, Kettering crawls through a hidden passage behind a sliding wall and confronts a final figure: an older expedition member, Professor Cole [Leonard Nimoy], who claims a “position of a much higher order.” The revelation confirms that the parasites are invading from inside the Earth itself, aiming to impose a utopia by erasing human conflict and strife. After the unsettling encounter, the possessed Cole disappears, and Kettering shoots the lurking sheriff to prevent further harm. Parasites then chase Kettering and Glenn outside.
In a daring gambit, Kettering devises a plan to force an electric connection using an abandoned truck. He threads an electrical wire from one end of a ravine to the other and intends to fire a harpoon from the cone to a distant high-voltage line, thereby completing a dangerous circuit that could destroy the intruders. Just as he prepares to execute the plan, Alice emerges from the cone, possessed once more, and refuses to move—she kills him with a pistol rather than letting him save everyone. With resolve, Glenn fires the harpoon gun, completing the circuit and sending a lethal jolt of electricity through the cone. The parasites inside are electrocuted, and Alice collapses. In the aftermath, Glenn and Elaine walk away from the site, sharing a quiet, relieved embrace as the danger in Riverdale subsides for now, leaving questions about what else lies beneath the surface.
