Bear with me: A Nazi scientist ships the immortal beating heart of the Frankenstein monster via submarine to Japan — Hiroshima, to be exact, which suffers its fate — later we find a young boy (called a “waif” in this movie) who has somehow either ingested or otherwise bonded with the heart — it’s REALLY off the wall. He mutates and begins to grow way beyond human size. Apropos of nothing, an underground monster named Barugon appears and the monsters battle. This movie is a real oddity. I’d never heard of it until this year. It’s weird.
Frankenstein Conquers the World, (released in Japan as Frankenstein versus Subterranean Monster Barugon (フランケンシュタイン対地底怪獣バラゴン Furankenshutain Tai Chitei Kaijū Barugon)) 1965