Frankenstein Conquers the World (1965)

At the end of World War II, Dr. Frankenstein is working with the Nazis and must pack up his creature's still-beating heart to keep it from falling into the hands of the advancing Allies. After Hiroshima, a doctor (Nick Adams, also seen in Monster Zero) and nurse working on the effects of radiation on the human body encounter a Wild Child who seems to completely lack in any civilized influence...

but he has an abundance of forehead.

As the Wild Child continues to grow exponentially, a reptilian creature (Baragon) emerges from underground. Baragon terrorizes the people of Japan as suspicion about the Wild Child grows. Both scientists and the public wonder if the Wild Child is, in fact, the Frankenstein Monster grown from the still-beating heart that made it to a hospital in Hiroshima before the bombing.

Their suspicions are confirmed when they find the creature's severed hand, still crawling around in his cell.

Baragon's attacks are mistaken for the work of the Frankenstein creature, so the military is called to take him down.

Everyone realizes their mistake when the Frankenstein creature takes on Baragon head-to-head.

Truly, a battle for the ages.

The two giant monsters in this movie are probably two of the worst in the entire Toho Studios lineup. One is basically a dinosaur dog with a unicorn horn nose.

And the other is a skinny kid with a giant forehead.

Sure, the other elements of the story are pretty straight-faced (the mood is a bit somber with so many mentions of radiation poisoning and the Hiroshima bombing), but there's not much getting around the terrible giant monsters in a movie that is about giant monsters fighting one another.

Definitely recommended for fans of 1960s Toho films, recommended with reservations for giant monster movie fans, and not recommended at all to anyone else.