A follow-up to the 1964 monster mash-up, Ghidorah, the Three-headed Monster, Toho's Invasion of Astro-Monster (1965, but released five years later in the U.S. as Monster Zero) continues to chart the big lug's transformation from Atom Age Menace to lumbering reluctant savior of the planet.
Starring American actor Nick Adams (who'd previously appeared in Toho's Frankenstein Conquers the World) with his voice dubbed into Japanese on the subtitled version (as with the Ghidorah DVD, Classic Media provides both English and Japanese versions of the flick), Astro-Monster takes place in the future year "196x," when a new planet has mysteriously appeared on the other side of Jupiter. A pair of World Space Agency astronauts named Glenn (Adams) and Fuji (monster movie regular Akira Takarada) travel to the new planet, which turns out to be inhabited by pale-faced aliens who live underground. (First indication that the planet is inhabited: our astronauts spy a trail of shoeprints in the planet's soil – proving that a decent pair of well-heeled footwear is appreciated throughout the galaxy.) Planet X, which looks a lot like the crappy asteroid soundstage in Michael Bay's Armageddon, appears to be under siege from King Ghidorah, last seen flying off into the solar system in his debut flick, and the aliens tell our heroes that they need the help of Earth's mightiest homegrown monsters to destroy the three-headed beast. If Earth is willing to let the X-ers "borrow" Godzilla and Rodan, the aliens promise to provide humanity with the cure for cancer.
Unfortunately, the people of Planet X – humanoids in snappy wraparound shades, vinyl spacesuits and helmets with a single antenna on top – don't really intend to honor their part of the bargain. Turns out they wanna conquer Earth for its water, and once they've captured Godzilla and Rodan, carting 'em through space in giant bubbles, they take control of all three monsters to use as weapons against the Earthlings. To this end, we get a prolonged monster bash on the barren Planet X and an even bigger monster fight back on Earth. More buildings (including a Mobil oil refinery) get trashed in this outing than in Ghidorah, but there also appears to be a greater number of visible wires this time out, too.
There's a subplot centering around a nerdy would-be inventor who has come up with a sonic device called the Lady Guard that basically seems to be just a loud noisemaker – though, of course, it plays a major role in freeing the monsters from evil alien control. A seductive lady alien (Kumi Mzuno) also enters the picture, romancing Adams' patsy astronaut and buying up the patent for the Lady Guard in a vague attempt to keep it from being used to foil the invasion plans. Our upstanding American hero gets to deliver a sterling speech about the importance of freedom and individuality to the unappreciative alien spy just before she gets vaporized by her fellow Planet X-ers for the crime of falling in love with a human.
But that's not the most emotional moment to come in this thoroughly silly epic, however. That belongs to the monsters, just before the aliens' flying saucers lift their big bods off the Earth. As the two big beasts stand and forlornly look across the ravaged Japanese landscape, one character notes, "They seem to resent us for leaving them behind. They cause trouble some times, but I can't help feeling sorry for them." A sentiment echoed by every child in the movie theater audience back in 1965-70, I'd wager . . .