Scientist Steve Karnes delivers a speech to a British scientific society, led by Professor James Bickford, about the dangers to marine life posed by nuclear testing. Before Karnes can return to the United States, a real-life example of his concern materializes when a fisherman in Looe, Cornwall is killed on the beach, and his dying word is "behemoth". Later, thousands of dead fish are washed ashore.
Karnes and Bickford travel to Cornwall to investigate the fisherman's death and, although his injuries seem to include radiation burns, they find no evidence of radiation on the beach. Then, Karnes goes to inspect a passenger ship found wrecked and badly damaged, with the loss of all on board. Back in London, the two scientists discover that samples of the dead fish contain large amounts of radioactive contamination. Karnes begins to suspect that the "behemoth" that the fisherman described seeing is some kind of large marine animal that has mutated as a result of being contaminated by nuclear testing.
The next attack is on a farm near the coast in Essex. A photo of the area reveals a huge footprint and paleontologist Dr. Sampson identifies the creature as a Paleosaurus, an aquatic dinosaur that emits an electric pulse like an electric eel. Karnes believes that the dinosaur is saturated by radiation, which is transmitted by the electric pulse, resulting in the burns seen on the fisherman 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 upstream, seeking out the shallow waters where it was born, but 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 River Thames, but the military officer believes that 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 when it gets too close. Soon, the behemoth surfaces in the Thames and capsizes the Woolwich Ferry.
Rising from the river, the creature rampages through the city, flattening cars and destroying buildings. Bickford and Karnes advise the military that the best way to kill the beast will be to administer a dose of radium, hoping to accelerate the radiation sickness that is already slowly killing it. While they prepare the dose, the behemoth continues its rampage, eventually plummeting through London Bridge back into the Thames.
Karnes and Bickford set their plan into action. An X-class submarine with Karnes on board 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, they succeed in firing the torpedo into the monster's mouth, and the behemoth roars in pain. Observers in helicopters later confirm 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.
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