Frankenstein Conquers the World (1965)

"A fearsome kaijin? A mad new antagonistic kaiju? A golden entertainment epic of Japan-U.S. collaboration!"

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.