Frankenstein Conquers The World (1965)

Starring Tadao Takashima, Nick Adams, Kumi Mizuno, Yoshio Tsuchiya, Koji Furuhata
Directed by Ishiro Honda
(actor & director credits courtesy IMDB.com)

The ever-beating heart of Frankenstein's monster grows a new body around it which swells to gigantic size, and while scientists and the military pursue it, a new monster emerges to threaten Japan. 

Although the film culminates in the expected giant monster fight, this is still a bit of a departure among Toho's kaiju productions, and a bit moodier and darker than I was expecting, but all for the good I think.  I loved the film's prologue, which traces the path of the preserved heart of the Frankenstein monster from a Nazi laboratory to a Japanese laboratory in Hiroshima, just before the city is bombed, which sets up the radioactive rebirth and growth of the monster.  This sequence is really well done, with a nicely designed German castle set, and good special effects for the dangers faced by the German and Japanese submarines transporting the heart.  Nick Adams, paired again with the lovely Kumi Mizuno, provides another stalwart lead, although he doesn't quite get to show as much emotion or personality as in Godzilla Vs. Monster Zero.  I liked Furuhata's characterization of Frankenstein's monster as he grows from a young boy to an immense giant, and freed from the need for a monster suit, the actor brings some lithe energy to his wrestling of the creature Baragon.  Many of Toho's best craftsmen worked on the film, from director Honda to composer Akira Ifukube, and special effects director Eiji Tsuburaya, and it shows.