Shigera Kawamura lives in the small village of Kitamatsu, on the outskirts of Kiyushu where he works for the coal mine as an engineer until he received a phone call that Mine Shaft No.8 was mysteriously flooded. Shigeru quickly ran into the mine shaft. All of his miners tells Shigeru that two miners (Yoshi and Gorou) were missing. Shigeru lead the other miners to investigate the flooded shaft and only found is the lacerated corpse of Yoshi, remain floating.
Shigeru's team removed Yoshio's body out of the mine and transferred to a doctor in the autopsy room. Shigeru warns everyone not to speak to the press until the police investigation begins. Outside of hospital, Shigeru meets with his fiance Kiyo, the sister of Goro. He comforts her that Shigeru sure Goro is innocence, not a killer
By nightfall, Shigeru soon arrives in Goro's house and comforts her, telling her that the officers that were killed were Goro's friends, and had no reason to kill them. Someone else, then, killed those men. As the two sit together, they've attacked by gigantic prehistoric dragonfly larva, Meganulon that came out of the mine and enters Kiyo's home, forcing Kiyo and Shigeru to flee.
Shigeru calls for help as the police rushes into the scene and investigate the house, but also attacked by Meganulon, forcing themselves to flee. As the villagers evacuate, Shigeru lead the group of police officers to pursue the prehistoric monster. Shigeru spotted Meganulon that climbing to the hilltop. They follow the Meganulon before open fire with their pistols, which ineffective against the insect's exoskeleton as two officers advanced too close. Shigeru watches two officers grabbed and killed by Meganulon's pincers before return to the mine. Shigeru and the polices discovered that two dead officers has the same deep gashes that found on Yoshio and other victims earlier, realizing that Meganulon is the real killer.
Later, Shigeru and a group of the metro-police head back into the mine to confront the insect monster and attempt to locate Goro, dead or alive. However, J.S.D.F. soldiers arrives near the village and escort Shigeru and the cops down into the mine shaft where water is lowered than expected. Shigeru enters the deepest part of the mine shaft and finally found the butchered Goro's corpse laying on the floor. Meganulon emerges and chases the men back up the mine shaft while soldiers fire the machine gun with no effect on armored insect and Shigeru release the mine cart which roll down the shaft and crashed into Meganulon's carapace, killing the giant insect larvae. Shigeru and the other venture back down into the shaft and collecting the Goro's body. Another Meganulon emerge from the cave entrance and attack Shigera as the ground begins to shake and cave-in, killing second Meganulon and trapped Shigeru in the cave.
Shigeru able to regain consciousness after having survived from cave-in and discovered that the underground cave is actually Meganulon's lair as he been surround by hundreds of Meganulons. Shigeru look up and discovered something horrified: a giant egg that was sitting inside the Meganulon's home as it began to stir. Suddenly, the egg hatched and enormous prehistoric winged pterosaur, Rodan emerged from the egg. Horrified, Shigeru watches as hatching begin to feed on every single Meganulons. With entire Meganulon colonies devoured, Rodan spread his wings and break through the earth surface and took ti the skies as Shigeru escape from the cave before he was rescued and taken to the hospital with suffering from memory lost and unable to recognized Kiyo.