Nuclear tests awaken a prehistoric monster.

American reporter Steve Martin comes to amid the rubble of Tokyo, a city almost completely destroyed the previous night. After Steve is rescued and taken to a makeshift hospital, he thinks back to a few days before: On the way to a new post in Cairo, Steve flies to Tokyo to visit old college friend Dr. Serizawa, a prominent theoretical thinker. In the waters below Steve's plane, a ship bursts into flames and sinks, moments after sending out a distress signal. At the Tokyo airport, Steve is asked by security officer Tomo Iwanaga if he noticed anything unusual during the flight. Tomo sadly reveals that he does not know what they are dealing with, then takes Steve to the communications room of the company that owned the sunken vessel. As Steve and Tomo observe, it is revealed that a rescue ship dispatched to the area of the disaster sank in a similar manner. After eight ships are obliterated, the Japanese begin to panic as news of the disasters are broadcast throughout the world. A meeting of Japan's top officials is held and Dr. Yamane, one of Japan's greatest scientists, testifies as Steve looks on. Yamane suggests that the investigation must begin on Odo, a sparsely inhabited island near where the ships sank. Meanwhile, on Odo, fearful natives rescue the sole survivor of one of the ships, but he dies after uttering a few hysterical words. Next morning, a helicopter carrying Japanese officials arrives on the island. Steve and Tomo, who are part of the group, learn from one terrified islander that he saw a monster. That night, Steve and Tomo watch a native ceremony that dramatizes the legend of a monster called "Godzilla," which islanders fear is responsible for the ship disasters. During the night, as a violent storm pounds the island, horrendous roaring sounds are heard as many of the villagers and their homes are destroyed. The next morning, officials bring some villagers back to Tokyo to testify before Japanese leaders about what they saw. Yamane then testifies, stating that they should not dismiss the native's comments and suggesting an expedition be formed. After his testimony, Steve, who had met Yamane on previous occasions, asks if he may come along on the expedition. Onboard the ship bound for Odo is Yamane's daughter Emiko and Ogata, a young officer with whom she is in love, even though she has been engaged to Serizawa since childhood. When the ship arrives at Odo, Yamane observes the destruction and discovers that a local well has been contaminated and is surrounded by radioactive footprints of a living creature. When an alarm sounds in the hills, everyone runs toward it then flee in terror as a huge, dinosaur-like creature appears on the horizon. Back in Tokyo, Yamane describes the Jurassic Age to officials, explaining that there was once an intermediary creature, called Godzilla by native peoples, that could live on both land and in the sea. Yamane then reveals his conclusion that, based on the presence on the island of Spendium 90, a bi-product of the H-bomb, the 400-foot tall Godzilla was resurrected due to "repeated experiments of H-Bombs." Later, Steve speaks on the telephone with his boss, George Lawrence, and relates that officials have decided to use sonar to locate Godzilla, then destroy it with depth charges. That afternoon, as Emiko is about to tell Serizawa that she wants to marry Ogata, Serizawa takes her into his laboratory filled with tanks of exotic fish. After Serizawa causes all of the fish in one tank to be completely destroyed, Emiko is terrified but agrees to his request that she not reveal to anyone what she has seen. Soon a large portion of the Japanese fleet is launching depth charges to try to kill Godzilla, even though Yamane feels that Godzilla should be studied rather than destroyed. After the day's bombardment, the Japanese celebrate Godzilla's destruction until revelers see the monster in Tokyo harbor. The army is mobilized, but when Godzilla rises from the harbor, machine gun fire has no effect. Thousands of frightened citizens try to flee Tokyo as Godzilla walks onto the land, crushing all buildings, trains and bridges in his path. Godzilla returns to the water after destroying the dock area, and the next morning officials work on a plan to stop the beast by entrapping it in the network of electrical power lines encircling the city. In anticipation of Godzilla's reappearance, and fearful of what might happen, Steve sets up his tape recorder to dictate a record of what he sees. Even though the city's electrical wires carry 300,000 volts of electricity, Godzilla is unaffected by their power. Heavy artillery fire is also useless, leaving Tokyo completely vulnerable. As Godzilla advances through the streets, impervious even to tank attacks, its incendiary breath causes huge fires to erupt, setting most of the city ablaze. Observing from the distance, Steve and other reporters are incredulous, but soon they, too, are trapped, forcing Steve to dictate "Steve Martin signing off from Tokyo, Japan." After destroying most of Tokyo, Godzilla returns to the harbor. As Yamane, Emiko and Ogata watch, the Japanese air force attacks Godzilla from the air, sending it back down into the water. In the morning, after Steve awakens in the hospital, Emiko and Ogata are at his bedside. When Steve wonders aloud what can stop Godzilla, Emiko tells him about what she saw in Serizawa's lab and reveals that Serizawa told her he accidentally came upon a means to destroy all oxygen in water, thereby killing all life. On Serizawa's insistence, she promised him not to tell anyone about the oxygen destroyer until he found a counter measure. Urged by Steve to try and change Serizawa's mind, Emiko and Ogata go to see the scientist. Emiko confesses that she broke her promise and begs him to use the oxygen destroyer against Godzilla. Angered, Serizawa rushes into his lab to destroy his experiments, but after knocking Ogata out in a scuffle, realizes that he and Emiko are right. Some time later, Steve becomes one of the witnesses who will observe Serizawa help Ogata destroy the beast. The two men dive off the ship and soon find Godzilla's resting place. Ogata rises to the surface, but Serizawa remains on the ocean floor holding the oxygen destroyer to ensure its success. After wishing Ogata happiness with Emiko, Serizawa unleashes the oxygen destroyer. Godzilla rises to the surface then flails back under the water as his flesh disintegrates. With Godzilla destroyed, those on the ship salute Serizawa for his sacrifice.