In 1958, two rival Japanese scientists, Ichiro Mizuki and Jiro Ikawa, travel to the remote wetlands of Papua New Guinea. Their goal is to harvest the blood of a legendary kaiju known as the Dai-Kaijū, a massive prehistoric creature resembling a Zeuglodon that has lived in the region for millions of years. The scientists believe the creature's blood holds the key to achieving immortality. Before Jiro departs, his sister Keiko offers him a good luck charm, but he dismisses it as superstition. Ichiro accepts the charm instead.

At the foot of Porogon Mountain, the men find the Dai-Kaijū and successfully extract a blood sample. Soon after, they are attacked by a hostile native tribe. Ichiro is wounded by a poison spear, and in the chaos, Jiro steals the vial of blood and leaves his colleague to die. Ichiro survives and manages to reach their boat, much to Jiro's horror. On the voyage back to Japan, Jiro tricks Ichiro into believing that injecting the raw kaiju blood will counteract the poison. Instead, the blood causes Ichiro to mutate into a giant reptilian creature, who then jumps into the ocean.

Jiro returns to Japan alone and is hailed as a hero for obtaining the blood. Soon after, Japan is attacked by a new giant monster, dubbed Raban by the public. Raban bears a striking resemblance to Godzilla, and the military is helpless against its rampage. Jiro secretly recognizes the creature as the mutated Ichiro, a fact he conceals from the public. He devises a plan to build a mechanical version of Raban, which he will pilot to destroy the threat.

Keiko, suspicious after seeing her good luck charm still hanging around the monster's neck, investigates Ichiro's past. She discovers that Ichiro is actually Jiro's long-lost brother, separated during World War II and presumed dead. As Jiro completes his mechanical Raban and prepares for battle, Keiko reveals the truth to him. Unable to kill his own brother, Jiro abandons his destructive plan and instead searches for a cure. With government funding, he develops a method to extract the Godzilla blood from Ichiro's system, successfully reversing the mutation and restoring him to human form.