In a charming little town in Pennsylvania, Steve Andrews and his girlfriend Jane Martin enjoy a sweet moment at a remote lovers' lane when they are suddenly interrupted by a meteorite crashing beyond the hill. Compelled by curiosity, Steve bravely decides to investigate, but it is the elderly resident Barney, played by Olin Howland, who stumbles upon the glowing rock first. His intrigue leads him to poke at the meteorite with a stick, which violently bursts open, releasing a bizarre gelatinous entity that latches onto his hand.
As Barney writhes in pain, a near miss with Steve’s car leads the young couple to rush him to Doctor Hallen's office, portrayed by Earl Rowe. After administering anesthesia to Barney, the doctor assigns Steve and Jane a critical task: they must uncover the meteorite's impact site and gather essential information. However, before they can complete their mission, Doctor Hallen faces a harrowing choice—amputate Barney's arm or risk losing him entirely to the relentless, cell-like substance consuming him.
The mysterious Blob, as it comes to be known, continues its inexorable advance through the town, devouring its residents one by one. Following Barney's disappearance, we witness the chaos as Kate, Doctor Hallen’s nurse, and then the doctor himself become victims of the growing menace. With each life the Blob claims, it expands and changes from a simple gelatinous mass to a menacing, larger, and more vibrant threat.
Steve and Jane return from their investigation just in time to witness the heartbreaking scene of Doctor Hallen attempting to escape through a window, only to be engulfed by the very creature he sought to evade. Frantically, the couple seeks help from the local police at the station, where Lieutenant Dave Barton and Sergeant Jim Bert dismiss their alarming claims as mere fabrications.
As night falls, Steve and Jane find themselves increasingly desperate, struggling to convince others of the lurking danger until they come across a mechanic who has already succumbed to the Blob's hideous appetite. Realizing the severity of the threat, Steve gathers his friends and community members in an effort to spread the word about the monster that has infiltrated their town.
Despite facing skepticism and scorn, a turning point arises during a midnight screening of Daughter of Horror at the Colonial Theater, where Steve unexpectedly finds support in Tony, who shares his fears about the Blob’s rampant destruction. The night spirals into chaos as the monstrosity continues its relentless onslaught, leading Steve, Jane, and her younger brother Danny to a diner filled with terrified patrons.
In a nail-biting standoff, they, along with the diner’s staff, face the greed of the ever-hungry Blob, which has grown even larger after consuming countless victims. Dave takes control of the situation, advising everyone to seek safety in the cellar while strategizing with Sergeant Bert to electrocute the being by taking down a high-voltage power line. Although this tactic lacks the desired impact on the creature, it leads to a fire ablaze beneath its gelatinous form, creating an opportunity for the diner owner to fight back with a fire extinguisher.
As Steve notices the Blob reacting to the cold, he realizes it is sensitive to low temperatures. He conveys this crucial revelation to Dave, igniting hope for the captured diners. Armed with this knowledge, Steve’s friends, local firefighters, and police form a united front, mounting a daring rescue operation to produce a cache of carbon dioxide extinguishers from their high school, lead by Henry Martin, Jane’s father.
With a united effort, they finally drive the Blob away from the diner and manage to rescue the five survivors. The authorities then devise a plan to encase the now-frozen monster and airlift it to the Arctic, where the cold will theoretically halt its advance without killing it. As Dave contemplates the implications of this plan, the film concludes with a chilling uncertainty, as the words "The End" drift into the air transformed into a question mark, leaving the fate of the Blob—and perhaps the town—uncertain as the credits begin to roll.
