American scientist Steve Karnes (Gene Evans) gives a speech to a British Scientific Society meeting led by James Bickford (André Morell) about the threat to marine life created by nuclear testing. Before he can return home, a fisherman in Cornwall is killed on the beach. His last word is Behemoth. Then, thousands of dead fish are washed ashore.

Karnes and Bickford head for Cornwall to investigate. The man seems dead from radiation burns, but there is no radiation on the beach. Karns studies a passenger ship found wrecked with the loss of all hands. Back in London, they discover that the dead fish definitely show signs of radiation. Karnes believes that the Behemoth is a large marine animal mutated by nuclear testing.

Another attack takes place on the coast of Essex. There is a huge footprint and paleontologist Doctor Sampson (Jack MacGowran) identifies a Paleosaurus, an aquatic dinosaur that emits electricity like an electric eel. Karnes thinks the Behemoth is saturated by radiation, thus the burns that it causes. Sampson thinks the creature will leave the ocean and head for the shallows where it was born, passing through London on the way.

Karnes and Bickford try to persuade the government to close the river Thames, but the military trusts its radar tracking system. The trouble is, the Behemoth seems to be invisible to radar. Sampson and others spot it from a helicopter, but it destroys the copter. The Behemoth surfaces in the Thames and capsizes the Woolrich Ferry. Climbing out of the river, it sets the city on fire and kills many of the people running through the streets. Bickford and Karnes suggest the military hit it with a dose of radiation, accelerating the radiation sickness that is slowly killing it.

The creature collapses part of the London Bridge with its weight and falls back into the Thames. An X-class submarine with Karnes on board is armed with a torpedo filled with radium. The Behemoth takes a bite out of the mini sub but they try again. They fire the torpedo into the creature’s mouth and hear it roar in pain. Helicopter observation shows that it has died. Getting in the car to leave, Karnes and Bickford hear of dead fish washing up on the U.S. East Coast.

The film was directed by Eugene Lourie and the special effects team was led by Willis O’Brien, who did them for King Kong. The script was written by blacklisted author Daniel Lewis James under the name Daniel Hyatt. If it sounds like the Beast from 20,000 Fathoms, it’s because the distributor wanted it that way instead of the amorphous radioactive blob in the early script. The stop-motion animation was shot in a Los Angeles studio and somebody’s garage and then optically integrated into scenes filmed in London. The screams of the people in London came from King Kong, including the screams of Fay Wray.

A woman drinking tea while listening to the radio was Jessie Robins, who played Aunt Jessie in the Beatle’s Magical Mystery Tour (1967) and the shot of a block of flats in London was the Beatles’ home in Help! (1965). The Behemoth crashes through the London Bridge in the same way as the Brontosaurus in 1925’s The Lost World. Director Eugene Lourie, who directed The Beast from 20,000 Fathoms, was displeased with the love-interest in that film and argued to keep this one scientific. Thus Leigh Madison, the fisherman’s attractive daughter, had little to do in the film. The creature was pretty good, but it’s size was a little inconsistent. The commentary of the DVD was wretched, featuring people who seemed to know nothing about the movie.