Well, with the second American Godzilla taken care of, I'd say it's about time to get back to the Japanese films, right? The next item in my list is Invasion of Astro-Monster, originally released in the U.S. as Monster Zero. A wide fan favorite Invasion of Astro-Monster was the first Toho film to marry two tokusatsu subgenres - the space adventure and the kaiju tale - into one movie, a combination which would soon flourish throughout the Showa era.
Invasion of Astro-Monster is often considered a a sequel to Ghidorah, the Three-Headed Monster but its connections to the previous Godzilla film are actually fairly slight, even by comparison to Toho's previous attempts at a shared continuity. The events of the prior film are never directly referenced (although seemingly relevant) and it would be easy enough to understand Invasion's plot without having ever seen its predecessor. That being said, Invasion is a natural progression from Ghidorah, both by featuring Ghidorah (the "Astro-Monster") as well as by continuing the evolution of Godzilla and Rodan into monster protagonists, after their initial run as villains.
Invasion of Astro-Monster is also arguably the last of the "classic" Showa films, before the series' budgets took a dive and both director Ishiro Honda and special effects supervisor Eiji Tsuburaya departed. Later Showa films would leave behind Honda's more somber tone and instead embrace the campiness for which kaiju films would later be known, something the director had consciously tried to avoid during his run (though not always successfully).
The story begins in the outer Sol system where an international team of astronauts (composed of the Japanese Fuji and the American Glenn) exploring a hitherto unknown moon of Jupiter (oddly referred to as Planet X). Descending to the moon's surface to examine it more closely the astronauts are unexpectedly abducted by its alien inhabitants. Although initially alarmed, the astronauts are assured by the aliens, known as Xians, that they were interred for their own protection as a destructive monster is en route and would have destroyed them if not for the Xians' intervention.
Meanwhile, on Earth, Testuo Torii (Fuji's sister's fiance) tries desperately to sell an alarm he's invented for personal protection. Although highly effective the sound it emits is extremely annoying and repels most potential buyers. However, Tetsuo eventually receives an offer from the World Education Corporation, represented by Miss Naimkawa, who says they're willing to offer a high price for the alarm and its designs. Overjoyed someone is interested in his device, Tetsuo agrees to sell it to WEC.
Back on Planet X the captive Glenn and Fuji witness the arrival of the "Astro-Monster," which reveals itself as none other than King Ghidorah, the same beast which had previously attacked Earth (and Venus). The Xian base goes into high alert as the space dragon launches its assault, while Glenn and Fuji are placed into a cone of silence.
After Ghidorah is repelled offscreen the Xians inform their human captives that they need Earth's help to repel the monster once and for all if their civilization is to survive. When the astronauts ask what they could possibly do to help the Xians reply that they would wish to borrow Earth's own kaiju, Godzilla and Rodan. In return, the Xians offer to share their medical knowledge with humanity, promising to cure all disease.
Not entirely sure whether or not they can trust the Xians Glenn and Fuji give the aliens a cautious "maybe," after which they are allowed to return to Earth. A special summit of the United Nations is called together, which quickly agrees to the proposal although the world's leaders are unsure where to find Godzilla and Rodan or how to provide them to the Xians. Nonetheless, a search is launched.
Meanwhile, Fuji and Glenn return to Earth, where they meet with Fuji's family and Glenn begins a relationship with Miss Namikawa. The latter leads to complications, however, when Glenn runs into an associate of Namikawa's he believes to be the Xian Commandant he met on Planet X. Suspicious that the Xians may have ulterior motives, Glenn asks Fuji whether Earth should really go through with the agreement, a question for which Fuji has no real answer. It probably bears mentioning at this point that Earth's actually been invaded twice already, by the Mysterians in 1957 and the Natal earlier in 1965.
As it turns out, Glenn's suspicions are well-placed. Namikawa's associate is the Commandant and the WEC is an Xian front. Namikawa herself is an Xian agent, tasked with acquiring Tetsuo's invention specifically for the purpose of sequestering it and destroying the designs. Furthermore, when Earth takes longer than hoped to provide Godzilla and Rodan, the Xians arrive in flying saucers to take the kaiju unilaterally, without Earth's permission. Further alarmed by this development, Glenn and Fuji decide to accompany the Earth embassy to Planet X in order to learn what it is the Xians are hiding.
Of the two subgenres, Invasion of Astro-Monster chooses to play with, the Mysterians-style alien plot is obviously predominant. Godzilla, Rodan, and King Ghidorah actually play a surprisingly small role in the story, with the real focus very clearly on the Xians and their suspicious activity. This isn't necessarily a problem, however, and the writing for that half of the story, by Shinichi Sekizawa, is competently done, though never quite as interesting as that for The Mysterians, penned by Sekizawa's colleague Takeshi Kimura.
The actual resolution to the story is somewhat comical, perhaps intentionally, but in a way that clashes with Honda's serious aspirations. Similarly, the Xians' rigid rationality and rejection of emotions, probably meant as a humanistic parable, feels somewhat hollow in comparison to similar attempts in Star Trek or even Marvel Comics. All the same, the writing in Invasion is entertaining and some of Sekizawa's best. The illusion is aided by a capable cast, including Nick Adams as Glenn and Kumi Mizuno as Miss Namikawa, both of whom had previously appeared together in Frankenstein Conquerors the World and obviously worked well as a pair.
Unfortunately the production values of Invasion of Astro-Monster do not hold up as well, even for their time. The alien spacecraft are no more convincing than they were eight years earlier in The Mysterians and the monster suits look remarkably ragged in comparison to prior films. This may, in part, reflect Toho's increasing cheapness; it's also the first Godzilla film to make extensive use of stock footage, a technique that would become increasingly common as the Showa era went on. Even Akira Ifukube's soundtrack suffers here; clearly fatigued, he reuses large portions of his prior work, with little new material to offer (though what's there is pretty good). The classic 1960s costume design for the Xians gets top marks though.
Weighing the film as a whole, I was somewhat disappointed by Invasion of Astro-Monster considering the level of affection held for it within the Godzilla fandom. But it's hardly a bad Godzilla film and if you're willing to tolerate some cheesy writing and cheap production values (which, really, if you're a kaiju fan you've got to) it's definitely enjoyable. I won't call it a classic, but it's a solid B movie. And sometimes that's all you need.