Frankenstein is portrayed as an innocent, childlike but still relatively intelligent creature, similar to the version of the monster that appeared in Universal Pictures' films featuring the character. Toho's Frankenstein is mute and unable to speak, but does appear able to understand human speech. He is also somewhat feral, but is not hostile or dangerous towards humans so long as they do not try to harm him. Frankenstein shows compassion and loyalty towards Dr. Sueko Togami, the scientist that raised him. Even after escaping from the laboratory, Frankenstein visits Sueko's apartment. Frankenstein also takes it upon himself to fight Baragon when the creature threatens Sueko.
Many years ago, a German scientist named Victor Frankenstein created an artificial human from the pieces of several corpses and reanimated it using electricity. Though the creature appeared to die at several points over the next several decades, it always managed to return due to its immortal heart. By 1945, a scientist in Frankfurt Germany, Dr. Riesendorf, was experimenting on the heart of Frankenstein's monster, only for Hitler's Nazis to seize it. The Nazis, hoping to use the heart's secrets to create immortal soldiers, shipped the heart to Japan in order to prevent it from falling into the hands of the Allied forces. The heart was taken to a Hiroshima laboratory for study but was lost and seemingly destroyed when the United States dropped the atomic bomb Little Boy on the city. Ten years later, a wild boy was discovered wandering the streets of the city. Scientists eventually captured and studied him, discovering that he was regenerated from the lost heart of Frankenstein.
Frankenstein vs. Baragon
Ten years after the bombing on Hiroshima, a wild boy was discovered wandering around the city Hiroshima. Scientists captured and studied him, discovering that he was regenerated from the heart of Frankenstein. Frankenstein was well fed by the scientists, and in response to the protein-rich food, began to grow incredibly fast, reaching a height of 20 meters with extraordinary speed, forcing the scientists to incarcerate him in a cell. However, Frankenstein managed to escape his prison, his hand actually snapping off when his chains became too tight. Frankenstein then fled to the Japanese countryside, sending all of Japan into a panic and being blamed for the disappearance of both livestock and people. It was later discovered that the people have been eaten not by Frankenstein, but by a new, burrowing kaiju named Baragon, who has eluded detection for quite some time. However, when Baragon attacked Frankenstein's former caretaker, Dr. Sueko Togami, the human-like monster attacked him. A vicious battle ensued, ending with Frankenstein apparently having killed Baragon by breaking his neck. Frankenstein, however, did not have long to savor the victory, for the ground underneath him, which had already been weakened by Baragon's burrowing, gave way and sent Frankenstein into the bowels of the Earth. Sueko asked if Frankenstein was killed, but one of her colleagues stated that Frankenstein could never die and would return again. Dr. Bowen, their other colleague, suggested that maybe it was best if Frankenstein did die, as a monster like him could never exist peacefully in this world.
War of the Gargantuas
It was later discovered that some of the immortal cells left behind in Japan by Frankenstein regenerated and took on lives of their own. One of these Frankenstein spawns, Sanda, was raised from childhood by a team of kind scientists before escaping to the mountains. The other, Gaira, grew up underwater surrounded by dangerous creatures, and became a violent and hateful beast.