No, it's not a horror film. It's another of Ishiro Honda's giant rampaging monster movies, known in Japan as Frankenstein versus Subterranean Monster Baragon. It gets duller as it goes along, but I liked the first half-hour.

We begin with Nazis in 1945 confiscating a scientist's work and making him throw a tantrum and smash up his own lab equipment. Confusingly, the Nazi officers are wearing British Royal Navy uniforms. You'd think someone on the production would have spotted this. Anyway, the Nazis carry their loot away to a submarine and it suddenly dawns on the audience that, hey, when a Japanese submarine shows up, they'll be the Nazis' allies. The stolen treasure is duly transported to Japan, where the great Takashi Shimura gets a cameo as "Axis Scientist" to explain what they've nicked.

It's the immortal still-beating heart of Frankenstein's monster, in a pot of blood. Now that's an unexpected genre mash-up. (Frankenstein's immortality is explained via a theory that until now I'd only ever seen applied to Dracula, i.e. he keeps getting killed and coming back in the next movie.)

A few (screen) minutes later, the war's over and we're looking at Hiroshima in the aftermath of the U.S. nukes. That's something else I didn't expect in a cheap and cheerful kaiju movie. Nick Adams (on loan from Hollywood and dubbed convincingly into Japanese) is an American scientist helping care for the survivors and do research into how to treat radiation poisoning. One of his patients (a pretty young girl) gives him a cushion she's embroidered herself. Adams thanks her, then discusses with his assistant (Kumi Mizuno) how long the girl has to live (not long) and how much incurable pain she's suffering (lots, with no way of relieving it). Not long afterwards, Adams and Mizuno will visit the girl's grave, although the English subtitles will pretend it's her parents' grave instead.

Again, my expectations are being confounded. These are rich story ingredients and a million miles away from the usual cheesy nonsense with rubber monster suits. We visit lots of famous Japanese locations. A mob of stampeding children make me laugh by finding a dead rabbit in their classroom.

Then there's Frankenstein's Creature, referred to throughout just as Frankenstein. (If we regard his creator as his de facto legal parent, this might conceivably be correct terminology.) I liked him. As with King Kong, the Japanese are trying to do a good job with this icon instead of just cashing in on cheap name recognition. He's a monster, but he's sympathetic. He's a scrawny Japanese caveman with super-growing powers that will by the end of the film see him grown to Godzilla-size, although disappointingly (and with no explanation) his clothes grow at the same rate. Naked Frankenstein would have been funny. Anyway, he doesn't really mean anyone any harm. He's a nice guy. It's just that he also happens to have dog-level intelligence, indestructibility and the understandable need to eat food. He can't speak, although he seems to understand what Kumi Mizuno wants when she speaks to him.

He's immune to radiation, obviously. He's immune to everything, including having his limbs cut off. (They'll stay alive while he regrows new ones, like Tomie.) This allows an unusual moment of empathy for a Frankenstein film, in which our heroes wonder what it must have been like for him to have grown up alone in a radioactive war-ruined city. We're given the image of a three-year-old Toddler Frankenstein, in the aftermath of the nuke with no one to care for him. What kind of childhood must he have had?

All this is great. I loved it. The first half hour isn't about rampaging monsters, but is instead about three compassionate scientists trying to study and help a man who just happens to be a flesh-eating Frankenstein's monster. Kumi Mizuno is his best friend. Unfortunately sooner or later, he's going to outgrow his cage and get out into the wild, whereupon the narrative becomes more conventional and dull. Scientists together with Frankenstein are fascinating. Separated, they don't actually have that much to do. Frankenstein goes on a walking tour of Japan and becomes unpopular without hurting anyone. Booo. Fortunately he seems to have worked out that being immortal and indestructible allows him to hide out underwater, since presumably he's no more vulnerable to being drowned than he is to radiation or bullets. Meanwhile Adams, Mizuno and their colleague Tadao Takashima go looking for Frankenstein and would (mostly) like to protect him, but this doesn't translate into anything interesting happening.

There's also Baragon, making his kaiju debut. He's a burrowing dinosaur with a heat ray. Imagine a cross between a triceratops and an ankylosaurus, but with bat ears, a goofy troll face and a big glowing spike on his nose. Baragon pops up and causes random destruction from time to time, some of which is quite cool, but it's clearly not his movie.

The narrative staggers on like this for quite a long time. To be honest, I'm not sure I could necessarily tell you the point of the film's final hour, except that it ends with a Frankenstein-Baragon wrestling match, which is funny. Well, unless you're watching the film's International Cut, which has Frankenstein fighting a giant octopus for no reason whatsoever. As Ishiro Honda explained, "The movie was made in co-production with an American company, Benedict Pictures Corporation. The bosses were so astonished by the octopus scenes from King Kong vs. Godzilla, they begged to include it into the screenplay, even in spite of logic. So we shot some scenes with the Giant Octopus but, in the end, they were left out of the picture." There's an octopus battle in the sequel, though, War of the Gargantuas.

No one does anything that could even remotely be called "conquering the world", by the way, but who cares about the American title?

The acting's fine, with the big surprise being Nick Adams (Oscar-nominated two years earlier), a few years before dying at the age of 36 from a prescription drug overdose. Astonishingly, we have a Caucasian who can act in a Japanese monster movie! He can say his lines, because he's dubbed. He's likeable. He'd once publically insisted that he'd never work outside America, but this was the first of four Japanese films he made in 1965-66 and everyone there liked him. (This is particularly true of his co-star Mizuno, with whom he may have had an affair.) They called him a "team player" and I'd agree that he's an asset to the production, although of course he's only doing a good (not brilliant) job in a monster movie.

Would I recommend this film? Nope. It's pretty loopy and theoretically there's a lot to admire in its wildly original take on the Frankenstein legend, but at the end of the day it's another monster movie with less depth than it had earlier seemed to be threatening. Its first half-hour is terrific, but then the scientists get sidelined and stop making much difference to the plot. Lots of screen time, but not much to do. Instead the film's building up to have Frankenstein fighting Baragon. Uh-huh. I'll admit that there's a laugh or two in the 1960s dancing and in the triumphantly goofy special effects (Baragon, that wild boar) that are happy to be by turns silly and genuinely impressive. However these are mere amusements.

The heart of the film is Frankenstein himself. Frustratingly, he's nearly great. I liked what happened at the finale between him and Takashima, if only because I thought someone was about to have his head bitten off, but there's just not enough dramatic meat for me to feel that Toho had done their unique Frankenstein justice. He has a fight. The end. There's still a lot I like here and it's far better than it might have been, but there's a far more powerful film in this material that started slipping away after the half-hour mark.