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During WWII, a human heart taken from Dr. Frankenstein's lab and is kept in Japan where it gets exposed to the radiation of the bombing of Hiroshima. The heart grows in size, mutates and eventually becomes an indestuctable humanoid. Later, a reptilian monster goes on a rampage. Eventually the Frankenstein creature and the reptile face off in a terrible battle. An experimental lab animal called a gargantua escapes from his captors and is suspected to be the creature that is killing people all over the countryside. But when the gargantua from the lab appears at the same time as the evil gargantua, the two begin to battle across Japan.
During WWII, Germans obtain the immortal heart of Frankenstein's monster and transport it to Japan to prevent it being seized by the Allies. Kept in a Hiroshima laboratory, it is seeming lost when the United States destroys the city with the atomic bomb. Years later a wild boy is discovered wandering the streets of the city alone, born of the immortal heart.
Gaira, a humanoid sea beast spawned from the discarded cells of Frankenstein's monster, attacks the shores of Tokyo. While the Japanese military prepares to take action, Gaira's Gargantua brother, Sanda, descends from the mountains to defend his kin. A battle between good and evil ensues, leaving brothers divided and a city in ruins.