A nameless survivor of the bombing of Hiroshima, this small boy would be given the name "Frankenstein" after the notorious scientist Victor Frankenstein. During World War II, Japanese scientists in Hiroshima acquired the still-living heart of the original Frankenstein monster from the Nazi scientist Dr. Liesendorf, only to seemingly lose it when the United States bombed the city. However a child survived the blast. Suffering from radiation poisoning and starving, he found the disembodied heart and ate it.
Soon thereafter, the boy, surviving due to the combination of nuclear radiation as well as the unique cellular properties of the heart, grew quickly to adolescence. Meaning no harm, his attempts to steal food nonetheless got him captured by the authorities, and he was placed in the care of visiting American scientist Dr. James Bowen and his colleagues, Dr. Yuzo Kawaji and Dr. Sueko Togami. The boy remained in Bowen's laboratory until he grew to adulthood and escaped into the countryside during an ill-fated press conference, when photographers' flashbulbs annoyed him. There he continued to grow, soon becoming a giant. In the meantime, strange deaths were occurring, for which Frankenstein was wrongfully blamed.
The true culprit turned out to be the burrowing dinosaur Baragon. Frankenstein fought Baragon and defeated him, but was then attacked by a giant octopus which dragged him into a nearby lake and drowned him. However despite his death, Frankenstein's legacy would live on in the form of two genetic offspring, his "sons" Sanda and Gaira.
An alternate ending to the film depicts Frankenstein falling into a fissure created by a sudden earthquake after killing Baragon.
For some time this was the official ending of both the Japanese and US versions of Frankenstein Conquers the World, with the "giant octopus ending" present only in international cuts of the film, with some debate as to which ending was the official one. However the footage with the octopus was restored for the film's Stateside DVD release, and the DVD's audio commentary posits that the "octopus ending" is the "normal one." It is certainly more canonical than the "earthquake ending" since it is the only explanation for how Frankenstein's cells would've wound up in the sea, leading to the creation of Gaira.