The Giant Behemoth is the titular main antagonist of the 1959 science-fiction monster film The Giant Behemoth.
The Giant Behemoth was first mentioned by the old fisherman Thomas Trevethan in his final word "behemoth". He and thousands of dead fish are found washed ashore on a Britain beach. Scientists Steve Karnes and James Bickford collect enough samples which lead them to assume radiation was the cause of death, and the "behemoth" is theorized to be some kind of large marine animal that also had been infected by radiation.
The next victims of the creature are a man, his son, and their dog. A photo of a huge footprint found at the scene is analyzed by Dr. Sampson, identifying the creature as a Paleosaurus, an aquatic dinosaur that could emit an electric pulse, similar to an eel. Karnes suggests that the Behemoth transmits the radiation through its electric pulses, explaining the burn marks found on the fisherman and the other victims.
The radiation is also discovered to be slowly killing the Behemoth. Dr. Samson says that the dying creature might be trying to leave the ocean to head upstream, seeking out shallow waters to where it was born. However, death by radiation poisoning may not come soon enough to prevent the creature from reaching London on the way, where it will likely cause havoc throughout the city.
Karnes and Bickford try to persuade authorities to close the River Thames, but the military believes by tracking the beast with radar tracking systems will be enough to prevent it from reaching the city. However, the Behemoth is invisible to radar, allowing it to easily swim past their trap. Dr. Sampson and other scientists spot the Behemoth from a Royal Navy helicopter, but even their radar could not track the beast. The Behemoth destroys the helicopter using radioactive emanations, before surfacing in the Thames while capsizing a Woolwich Ferry.
Rising from the Thames, the creature begins its attack on the city, flattening several cars and knocking over buildings onto fleeing citizens. Karnes and Bickford advise the military not to use bombs against the Behemoth, as blowing up the creature would just spread the radiation further and result in more fatalities. Instead, they propose to create a dose made of radium in hope to accelerate the radiation sickness that is already killing the Behemoth. As they prepare the radium dose, the Behemoth finishes its rampage by destroying electric towers and plummeting through London Bridge back into the Thames.
As the Behemoth continues to swim upstream the Thames, Karnes and Bickford set their plan into action, a mini-sub carrying a torpedo filled with radium in pursuit of the monster. During an initial pass, the Behemoth bites the mini-sub, but Karnes convinces the submarine captain to have another go. The sub manages to fire the torpedo into the monster's mouth, causing it to roar in pain as the radium accelerates the radiation level. Steam rising from the water indicates the monster's demise. However, despite the Behemoth being gone for good, Karnes and Bickford hear on the radio about dead fish washing up on the eastern shores of the United States, implying that the Behemoth may not be the last of its kind.