American scientist Steve Karnes (Gene Evans) delivers a speech to a British scientific society, headed by Professor James Bickford (AndrÃ© 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 foot-print 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. The radiation is also slowly killing the dinosaur. According to Dr. Samson, the dying creature will leave the ocean depths to head up stream, seeking out the shallow waters where it was born; unfortunately death by radiation may not come soon enough to prevent the creature from wreaking havoc on London along the way.
Karnes and Bickford try to persuade authorities to close the 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.
Rising from the Thames River, the creature attacks the city, flattens several cars, and knocks a building over onto fleeing citizens. Bickford and Karnes advise the military on how to destroy the beast: bombs are out of the question, because blowing the creature to pieces would just spread the lethal radiation farther, resulting in more deaths. The two scientists propose finding a way to administer a dose of radium to the behemoth, hoping to accelerate the radiation sickness that is already slowly killing it. While they prepare the dose (which takes time because radium must be handled carefully), the behemoth continues its rampage, destroying electric towers and eventually plummeting through London Bridge back into the Thames.
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.
What is Dr. Sampson's job?
paleontologist