A powerful storm is raging in the dead of night, and a transport vehicle carrying uranium is forced to stop due to a landslide blocking off the road on the mountain pass they were traveling on. As the driver tried to contact the transport’s headquarters, a second landslide from the heavy rain comes crashing down onto the vehicle, sending it over the edge of a cliff and into the sea below.
The storm is the result of an intense typhoon striking Japan, leaving hundreds dead and several hundreds more without homes. Reports from the survivors begin to pour in to the City News Division, leaving every reporter on staff manning the phones to take down any new information regarding the storm. One reporter, Goro Ryouji, nicknamed the “Suppon”, arrives late, much to the anger of his boss, but he reveals that he’s following up on the transport vehicle’s crash. He also lets his boss know that the news division’s competitors don’t know about this yet, so Goro has an exclusive scoop at the time.
The next day, the National Atomic Energy Center and the police have begun to investigate the area near where the vehicle went off the edge. Detective Yamato arrives to ask Dr. Ukyo if there have been any readings of radioactivity, to which the doctor nods and wonders if the vehicle was completely washed to sea. Goro soon arrives, revealing his information on both Yamato and Ukyo before revealing that he knows about the special uranium that it was containing, Uran X. As the three talk, they begin to hear strange roaring noises near the mountainside, which one of the police officers refers to as the “Tengu’s Great Trumpet Shell”, a large cave that funnels air in such a way to produce the sound.
Goro rushes into the cave to explore it, only to run back out in a panic exclaiming that something has appeared. A strange light appears from the tunnel, which Goro says was a one-eyed monster, but as the four look on in fear, the light goes out, and a young boy with a flashlight, Monta, comes out of the cave. Yamato asks where Monta found the flashlight, and after some coaxing, it is revealed that it was from the transport vehicle and that he found it amongst the rocks.
Goro and the others suddenly begin to hear a loud noise behind them, likewise noticing that one of the Geiger counters pointed out towards the ocean is reading levels off the scale. As they look out towards the ocean, a large, reptilian creature slowly breaches the surface.
The next day, news of the creature begins sweeping the news, and Goro and Yamato, on their way to the Atomic Research Center to see Dr. Ukyo, begin to wonder why the monster has decided to appear now. As they’re making their way towards the center, they are forced to stop when a dog sitting on the road refuses to move out of the way. Goro tries to coax the dog off to the side when a young woman appears and tells Goro that the dog won’t let them pass because it has the common sense to know that smoking isn’t allowed at the center. Once Goro puts out his cigarette, the dog walks off to the side of the road, and Yamato, joking that Goro lost, drives on, leaving the reporter to chase after him on foot.
Dr. Ukyo arrives and begins to tell him that he has information regarding Agon, the name he has given to the monster. He reveals that it is a mutated dinosaur from the Jurassic period. Believing it to have been dormant under the ocean, Ukyo suggests that the mutation is a result of nuclear bomb tests in the Pacific. The woman from the road, Satsuki Shizukawa, appears, and Dr. Ukyo reveals that she is his research assistant. Just then, she receives a phone call reporting that the center’s uranium storage checking meter is showing an abnormality.
With no issues with the uranium fuel, Goro, Yamato, Dr. Ukyo, Satsuki, and several other scientists make their way towards the beach and begin to pick up high radioactive readings. Dr. Ukyo sends Satsuki to return to the center to alert the authorities, but as Agon appears and begins to tear through the forest, Satsuki falls and her leg gets pinned under a tree. As Goro and the others reach the Research Center to begin to evacuate the facility, he realizes that Satsuki hasn’t arrived. Satsuki tries desperately to pull her leg free from under the tree, but as Agon approaches, the weight of the creature opens up a large fissure in the ground, and the ground underneath her gives way, causing the scientist to fall down into it.