The movie's plot involves a meteorite which falls near the small town of Downington, Pennsylvania. An old man pokes it open with a stick and finds a small, jelly-like mass inside. It slides up the stick and onto his hand, and turns out it is a living parasite. Two teenagers, Steve Andrews (McQueen) and his girlfriend Jane Martin (Corsaut) are out on a drive when they pick him up and take him to the doctor's office. There it consumes and dissolves the old man completely and then the nurse and eventually the doctor. Growing bigger with each victim, the Blob continues its rampage by consuming a mechanic and then a janitor in Steve's father's grocery store. It follows Steve and Jane into the stock room at the back of the store, but won't enter the freezer. Then it goes to the Colonial Theater during a midnight showing of a horror movie and eats the man running the projector, and then oozes through the wall at the back of the theater from the projection room. People run screaming as the Blob eats more victims, and then it emerges from the theater and engulfs a diner, where Steve, Jane and several others are trapped after fleeing there. Eventually, the heroes learn that the Blob can't stand cold, so they hit it with carbon dioxide fired from fire extinguishers grabbed from the local high school, which freezes it into a harmless block of ice. Then it is shipped to the Arctic.
Many critics view the film as a concise and well-spoken metaphor for the Cold War and condemnation of liberal communist support. The old man devoured at the beginning of the film by the red-colored Blob clearly represents the old European powers absorbed by the Soviet Union, while the unlistening and power-mad authority figures that refuse to listen to the well-reasoned and logical warnings of the main protagonist represent the liberal institutions that placed the Soviets in such a position of power. The Blob spreads and devours at will while the liberal powers-that-be do nothing, until they are helpless to stop it. It is only by the intervention of two upstanding and fair-eyed youths (fine examples of the rising youth culture whose hatred of Communism prevailed in the 1980s) that the Red Menace is foiled and exiled to a frozen wasteland (one of the easiest and clearest examples of cinematic symbolism, a representation of Siberia) so that right and humanity might prevail.
A sequel, Beware! The Blob, was released in 1972 and a remake of the original was released in 1988.
- However, Jack Harris, the man who sold the idea of the film to Good Times Films, described it as "...an investment. A $130,000 investment that turned a $10 million profit." The studio's resident director, Irvin Shortess Yeaworth, saw the film as a Biblical parable; the story being about God’s wrath upon evildoers