In London, American marine biologist Steve Karnes speaks at conference on atomic weaponry where he details his theory that the particles from numerous atomic explosions around the world have contaminated the oceans with potentially disastrous effects. Soon after in a small fishing village in Cornwall, fisherman Tom Trefethen and his daughter Jean reach the cove after a day’s fishing. When Tom does not arrive home later for dinner, Jean goes to the nearby pub in search of him. Fellow seaman John offers to help Jean locate her father and they return to the beach, where they find Tom covered with ghastly burns. Before dying, Tom mutters that he was attacked from the sea by a behemoth. After the funeral, John seeks to comfort Jean and as the couple walk by the cove, they are startled to come upon thousands of dead fish strewn along the beach. John then spots a strange white mound wedged behind a rock but when he reaches to touch it, he is severely burned. A few days later in London, Steve overhears a news report indicating that fishing has ceased in Cornwall after the discovery of the dead fish. Alarmed, Steve cancels his return trip home and contacts physicist Prof. Bickford, who relates the more serious details of Tom’s death and an additional reported sighting of a sea creature. Bickford allows Steve to accompany him to Cornwall to investigate, where the men discover most of the dead fish have been washed out to sea, or burned by the townspeople. After they speak with a fisherman who describes witnessing a glowing light over the water, John takes them to the doctor who examined Tom’s body. The doctor describes unusual burns but admits he did not feel it necessary to conduct a post mortem. The doctor observes that Tom’s burns match those on John’s hand and offers to examine him. Later, John takes the men down to the beach. Steve is puzzled by the absence of radiation readings in the area, but requests samples of fish from all along the British coast. Back in London, Steve conducts tests on the fish and is startled when one particular specimen contains a glowing, white object inside it. The fish is also revealed to be thoroughly contaminated by radiation. Although Bickford notes that the fish was picked up off the Essex coast, miles away from Cornwall, and expresses doubt that it is related to Tom’s death, Steve insists on investigating further. With Bickford’s assistance, Steve hires a boat to patrol the waters off Essex. Despite a thorough search, Steve finds no indication of radiation. As Steve and the pilot begin their return journey, however, the radiation detector abruptly reacts and through the fog Steve sees a strange, unidentifiable shape rise up out of the water, then move with astonishing speed. Bickford summons Steve to the remains of a steam ship on an Essex beach. At the site, Steve concurs the ship has suffered extreme radiation damage, but the scientists are puzzled by the complete absence of survivors. In London, Bickford and Steve then meet with an admiral from the Royal Navy to discuss the unusual destruction of the ship. Steve reveals that the white mass on the contaminated fish has been determined to be the stomach lining of an unidentified sea creature and suggests it could be responsible for the ship’s destruction. When Bickford agrees, the admiral orders that all international navies be placed on alert. Soon after, Bickford and Steve receive a report about the destruction of an entire farming village outside London-on-the-Thames and also receive a startling photo of a giant footprint. Based on the footprint type, the men take the photo to Britain’s most esteemed paleontologist, Dr. Sampson, who immediately identifies it as belonging to a prehistoric plesiosaurus, which had electrical properties similar to that of an eel. When Sampson learns that the creature was seen on the Essex coast, he suggests it is heading toward the fresh water of the Thames River in order to prepare to die. Excited by the possibility of coming in contact with an oversized, long-extinct creature, Sampson insists on joining the investigation. Steve and Bickford return to the admiral to explain how the creature’s natural electrical capacities allow it to project the radiation that has contaminated it. With the assistance of the military, Sampson tracks the creature by helicopter, but is attacked and destroyed by the beast. London is then thrown into a panic when the beast turns up in the Thames and destroys a ferry, killing several passengers. As the army oversees the evacuation of families all along the Thames, a plan is drawn up to destroy the creature. Steve and Bickford warn that should it be blown up, the radiated body parts could contaminate the entire city. Steve suggests that speeding up the creature’s own radiation poisoning by arming a torpedo with a warhead of pure radium would allow them to bury the carcass safely. While the delicate and powerful radium torpedo is being assembled, the creature goes on a rampage, coming inland and attacking various locations. When the creature stomps on London Bridge, it collapses, plunging the beast back into the river. Steve boards the submarine sent to eliminate the creature and after a tense chase, the torpedo is successfully launched, killing the sea creature. After docking back at port, Steve joins a relieved Bickford in time for the men to hear a report of several hundred dead fish washing up on the shores of America.