Far north of the Arctic Circle, a nuclear bomb test, dubbed Operation Experiment, is conducted. Prophetically, right after the blast, physicist Thomas Nesbitt muses, "What the cumulative effects of all these atomic explosions and tests will be, only time will tell." Sure enough, the explosion awakens a 10 meter tall, 30 meter long fictional carnivorous diapsid known as the Rhedosaurus, thawing it out of the ice where it had been hibernating for 100 million years.
The Beast starts making its way down the east coast of North America, sinking a fishing ketch off the Grand Banks, destroying another near Marquette, Canada, wrecking a lighthouse in Maine, and crushing buildings in Massachusetts. The Beast eventually comes ashore in Manhattan, and after tearing through power-lines attacks the city. The Beast's rampage causes the death of 180 people, injures 1,500 and does $300 million worth of damage.
Arriving on the scene, the military troops of Col. Jack Evans, blast a bazooka hole in the Beast's throat and drive it back into the sea. Unfortunately, it bleeds all over the streets, unleashing a "horrible, virulent" prehistoric germ, which begins to contaminate the populace, causing even more fatalities. The germ precludes blowing the Beast up or burning it, lest the contagion spread. Thus it is decided to shoot a radioactive isotope into the Beast's neck wound with hopes of burning the Rhedosaurus up from the inside, killing it.
When the Beast comes ashore and attacks the Coney Island amusement park, military sharpshooter Corporal Stone takes the potent radioactive isotope launcher,(it is the only one of its kind outside of Oak Ridge so he can't miss), and climbs onboard a rollercoaster. Riding the coaster to the top of the tracks so he can get to eye-level with the Rhedosaurus, he fires the isotope into the Beast's wound. The Beast lets out a horrible scream and crashes to the ground dead, with the surrounding park ablaze.