After conducting a secret experimental nuclear blast for the New York Atomic Energy Commission in the Arctic Circle's Baffin Bay, professors Tom Nesbitt and George Ritchie proceed to the blast area to observe the results. The two men separate to complete their tasks before an oncoming blizzard hits. Ritchie is soon buried in snow when a giant prehistoric reptile emerges from underground and causes an avalanche. Tom, responding to Ritchie's distress flare, is unable to find him, but sees the creature just before he is wounded and rendered unconscious. After being rescued by the U.S. Army, Tom is sent to a New York hospital to recuperate. There Tom's description of the reptile is explained away as a stress-related illusion and ignored, and Tom, himself, reluctantly comes to believe the diagnosis. However, when he reads in a newspaper that a ship sailing off the Grand Banks of Nova Scotia has been sunk, reportedly by a sea serpent, he leaves the hospital to enlist the help of Thurgood Elson, the dean of the department of paleontology at a local university. However, Elson is skeptical of Tom's theory that the heat of a nuclear blast freed a reptile that has been hibernating since the Mesozoic era. Elson's assistant, Lee Hunter, is open to the idea, and after a second ship is reportedly attacked near Marquette, she contacts Tom and asks him to look at several sketches of prehistoric animals. At her apartment, Tom identifies one of the sketches as looking like the creature he saw. At her suggestion, Tom tracks down George LeMay, the captain of the second ship, who is at first reluctant to risk further ridicule by talking about his experience. Tom gains his confidence and asks him to look over Lee's sketches. When LeMay picks the same sketch as Tom, Tom again requests Elson's help, this time successfully. Elson is fascinated that the identified sketch is a rendering of a rhedosaurus and explains that a fossil field in the Hudson Submarine Canyon, which is located 150 miles from the New York coast, contains the bones of several of these animals. Meanwhile, a lighthouse in Maine is destroyed by a giant reptile. While on a date with Lee, Tom is paged by the Coast Guard and told that another mysterious animal has been sited on a coastal farm in Massachusetts. Elson theorizes that the beast is following the Arctic current toward the Hudson canyon, the place the beast considers home. Excited by the possible discovery of a living specimen of his life's work, Elson arranges to go down into the canyon in a Coast Guard diving bell. However, as Tom and Lee wait for him in the Coast Guard ship, Elson, a Coast Guardsman and the diving bell are devoured by the creature. While Tom and Lee mourn for Elson, the giant creature shows up at a New York pier and proceeds across town, killing people with each step. As the police are unable to contain the beast, the National Guard is called in to set up a barricade. Sharpshooters try to shoot the animal between the eyes, but cannot penetrate his eight-inch skull. The Guardsmen manage to wound him with bazookas, but soon the hospitals are reporting that the beast is leaving a virulent disease in his wake and that contact with his blood is deadly. Shell fire and flame throwers are disregarded as a means of defense, for fear of spreading the beast's blood particles in the air. As the reptile heads for Coney Island, Tom suggests shooting a radioactive isotope into his wound, which would kill the beast, as well as the deadly bacteria. An isotope is brought to the scene and, as there will be only one chance, the best Guardsman sharpshooter is chosen to deliver the shot. He and Tom ride to the top of the roller coaster, where the sharpshooter hits his target. As the beast writhes in pain, he becomes tangled in the roller coaster and Tom and the marksman quickly climb down the structure to safety, where Lee and other Guardsmen await them. A fire erupts as the beast dies.