Frankenstein vs. the Subterranean Monster
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.