Originally a story about an amorphous blob of radiation, the script was changed at the distributor's insistence to a pastiche of The Beast from 20,000 Fathoms (1953), though elements of the original concept remain in the early parts of the film and in the "nuclear-breathing" power of the titular monster.
American scientist Steve Karnes (Evans) delivers a speech to a British scientific society, headed by Professor James Bickford (Morell), about the dangers to marine life posed by nuclear testing. Before Karnes can return to the U.S., a real-life example of his concern materializes when Tom Trevethan (Henri Vidon), an old fisherman, receives a lethal dose of radiation; his dying word is "behemoth". Later, thousands of dead fish are washed ashore.
Karnes and Bickford investigate the beach where the old man died, collecting samples which prove that radiation was the cause. Karnes begins to suspect that the "behemoth" that the old man described is some kind of large marine mammal that has been infected with radiation.
A man, his son and their dog are the next victims of the creature. A photo of the area reveals a huge footprint of some prehistoric animal. Dr. Sampson (Jack MacGowran), a paleontologist, identifies the creature as a "Paleosaurus", an aquatic dinosaur that emits an electric pulse, like an eel. Karnes believes that the dinosaur is saturated by radiation, which is transmitted by the electric pulse, resulting in the burns that killed the fishermen and other victims.
Karnes and Bickford try to persuade authorities to close the River Thames, but the military believes their radar tracking systems will be enough to detect the behemoth and prevent it from getting near the city. Unfortunately, the dinosaur appears to be invisible to radar. Dr. Sampson and some other scientists spot it from a Royal Navy helicopter, but the radar equipment tracking the helicopter sees no sign of the beast, which destroys the helicopter with its radioactive emanations. Soon, the behemoth surfaces in the Thames and destroys the Woolwich Ferry by capsizing it.
Karnes and Bickford set their plan into action. A mini-sub with Karnes carries a torpedo filled with radium into the Thames in pursuit of the monster. During an initial pass, the Behemoth takes a bite out of the mini-sub, but Karnes convinces the submarine captain to have another go. This time the sub fires the torpedo into the monster's mouth.
The behemoth roars in pain as the radium accelerates the high radiation levels that were already bringing about its death. Observers in helicopters see steam rising from the ocean, indicating the monster's demise. As Karnes and Bickford climb into a car to leave the area, they hear a radio report of dead fish washing up on the eastern shores of the United States.
- Gene Evans as Steve Karnes
- André Morell as Prof. James Bickford
- John Turner as John
- Leigh Madison as Jean Trevethan
- Jack MacGowran as Dr. Sampson, the Paleontologist
- Maurice Kaufmann as Mini Submarine Officer
- Henri Vidon as Thomas Trevethan
- Leonard Sachs as Scientist
The live-action scenes for The Giant Behemoth were filmed in Great Britain, including London. The model-animation special effects were shot in a Los Angeles studio, where they were also optically integrated with live-action footage.
In an odd connection between O'Brien and his most famous creation, stock screams that were used in King Kong can be heard in the scenes where the creature attacks the ferry and when it invades London.
In Video Movie Guide 2002, mention of the stop-action animation was made but with the proviso that "the film monster wasn't bad but [Willis O'Brien] was clearly working with a low budget".
American film critic Andrew Wickliffe considered the preamble to the appearance of the monster in The Giant Behemoth to be more interesting than the rampage that follows, writing, "I'm not sure the British are really suited for giant monster movies. No offense to the Brits, but watching a bunch of folks stand around and keep the stiff upper lip while radioactive monsters from the deep attack London isn't too much fun.".
Regarding a later release of the film in a package with other science fiction features, film reviewer Glenn Erickson observed that The Giant Behemoth was derivative, speculating that "... director Eugène Lourié apparently instructed writers Robert Abel and Alan Adler to repackage his original The Beast from 20,000 Fathoms, copying whole scenes and situations. The structure and script are almost a verbatim clone, right down to the dotty paleontologist".