Picking up fifteen years after the events of the first movie The Blob, an oil pipeline layer named Chester (Godfrey Cambridge) returns to his suburban Los Angeles home from the North Pole, bringing with him a small sample of a mysterious frozen substance uncovered by a bulldozer on a job site. Prior to taking the blob to a lab to be analyzed, he places the storage container with the substance in his freezer, but he and his wife (Marlene Clark) accidentally let it thaw, releasing "the Blob". It starts by eating a fly, then a kitten, Chester's wife, and then Chester himself (while, in an intentional anachronism by the film makers, he is watching a television broadcast of the film The Blob).
Lisa (Gwynne Gilford), a friend, walks in to see Chester being consumed by the Blob. She escapes, but cannot get anyone to believe her, not even her boyfriend Bobby (Robert Walker Jr.). Meanwhile, the rapidly growing creature quietly preys upon the town. Some of its victims include a police officer and two hippies (Cindy Williams and Randy Stonehill) in a storm drain, a barber (Shelley Berman) and his client, transients (played by director Hagman, Burgess Meredith and Del Close), a Scoutmaster (Dick Van Patten), a farm full of chickens and horses, people in a gas station, and a various townspeople who turn up "missing".
At one point, Lisa and Bobby find themselves trapped in Bobby's truck with the creature attempting to find a way inside. In the panic, the truck's air conditioning is accidentally switched on and the Blob retreats because of its vulnerability to cold.
The now-massive blob then invades a bowling alley and a skating rink (consuming dozens more people in the process). It is finally stopped when Bobby activates the rink's ice mechanism, freezing it. While the frozen blob is being filmed by a television crew, one of the crew's bright lights is positioned on the ground, melting a small portion of it, which oozes toward the sheriff and envelops his feet as he is speaking on camera to a nationwide television audience.
Larry Hagman previously directed episodes of I Dream of Jeannie and The Good Life and went on to do the same for several episodes of Dallas and In the Heat of the Night (the only series he directed for that he never acted on). This would be his only feature film as a director. To cast the film, Hagman recruited friends from the motion picture industry, (some of whom were literally neighbors of his in Malibu, California, including Burgess Meredith and Carol Lynley) who were asked if they would like to be "blobbed". Gwynne Gilford was cast as the lead via the traditional auditioning process, while Robert Walker Jr. was an early hire by Jack H. Harris himself.
Budgeted at slightly more than the 1958 version at $150,000, filming began in the spring of 1971, primarily on the property of a horse stable and ranch home in Diamond Bar, California, as well as in Pomona, California, both 30 miles east of Los Angeles. The ice skating rink scene (but not the bowling alley, which was a separate location) was filmed at the former Rollerdrome on Washington Boulevard in Culver City, California immediately prior to the building being torn down in August, 1971 to make way for Tellefson Park. The party scene was filmed in a loft in Venice, California. In an interview in Fangoria magazine, screenwriter and first time producer Anthony Harris stated that a good portion of the filmed material was improvised on the set, and that his script was ignored. While in production, Harris was also in the process of preparing a sequel, Curse of the Blob, but these plans were never implemented.
Contrary to the original 1958 film, in which the Blob was largely portrayed by gallons of dyed red silicone (which repeatedly needed to be stirred to maintain the consistent red color), the Blob in "Beware! the Blob" was mostly created from a red-dyed powder blended with water to form the desired consistency. The Blob was alternately created from other materials as well, including a large red plastic balloon, semi-transparent red plastic sheeting illuminated with a backlight, and a large rotating red drum of hard red silicone placed in front of the camera lens.
Renowned cinematographer Dean Cundey, who would later go on to be a cinematographer on such films as Halloween, The Thing, the Back to the Future series and Jurassic Park, worked on Beware! The Blob as one of the three special effects technicians (alongside supervisor Tim Baar and Conrad Rothmann) responsible for creating the blob effects. Cundey was also the camera operator on second unit shots of the blob eating the fly, the kitten, etc.
Beware! The Blob was intended as a December 1971 release, but was held off until June 1972 to capitalize on the summer movie audience. An exceptional film marketer, Harris paired the movie with other films he held the rights to (notably Equinox), and renamed the film Son of Blob in some markets as a test. The film premiered on television in 1974.
In 1982, the film was re-issued with the tagline "The film that J.R. shot!" in an attempt to capitalize on the success of Hagman's television series Dallas, using a twist on the show's popular meme, "Who shot J.R.?"
As Son of Blob together with The Blob, the film was released on DVD by Umbrella Entertainment in September 2011. The DVD is compatible with all region codes. "Beware! The Blob" was transferred to HD in its correct 1.85 ratio and released on Blu-ray by Kino Lorber on September 20, 2016.
Film historians Kim R. Holston and Tom Winchester considered the film was: "Now viewed as a relic of mid- to late-hippiedom ... overall, there's some tension, and some nods to the predecessor."