It's dull. It's 1950s SF. I deserve everything I get.
It's earnest, albeit in a confused way that ends up being accidentally subversive. Like much Japanese SF of this era, it's much higher-minded than comparable Western films. However its story of peace and universal brotherhood means that the film has no villains and no action, while it's also nearly as stupid as its Hollywood brethren. In particular its aliens are retards.
I'll give a quick run-down of the plot. Aliens from the planet Pyra have been visiting Earth for 4,000 years, watching us with increasing concern. They think we're on the point of destroying ourselves, you see. Isao Yamagata has discovered an explosive formula of such power that "even the H-bomb in comparison is a toy", but they insist that he should destroy his formula and never use it, because "energy used destructively is evil".
So these aliens have come to warn us against nuclear energy, right?
Actually, no. Some time later it turns out that there's a planet on a collision course with Earth... unless it's a sun. I suspect some confusion from the translators over that one. Anyway, we're going to get splattered unless we use all our explosive power to divert Planet R, as it gets called, and furthermore if Earth is destroyed then so will be Pyra. It's our twin planet, you see, locked in the same orbit but on the other side of the sun so we can never see it. This seems like the kind of thinking that mistakes outer space for railroad tracks, but we'll let that pass.
So in other words, these anti-nuclear pacifists need us to save their arses with our nukes. The film never comments on this. It also never comments on the fact that Earth's entire nuclear arsenal proves useless (nice irony) and in the end, the only hope for both planets lies in precisely that super-formula that they told Yamagata to burn without reading.
Let's look at the aliens' thinking in detail, shall we? Their plan to save two planets is:
(a) persuade mankind to use up its entire nuclear arsenal by visiting the one nation guaranteed not to have any,
(b) fly around randomly for a while, terrifying random passers-by and making not the slightest effort to communicate,
(c) impersonate a nightclub singer with amnesia and say nothing to any scientists except "super-explosive formulae are evil, so destroy your notes". However our heroine does attract attention by walking through walls and leaping ten feet in the air while playing tennis.
(d) tell mankind about the threat, then put your feet up and do nothing for a month.
(e) when Yamagata (i.e. the only person who knows the super-formula) gets kidnapped, only remember after that month's gone past that you'd planted a homing beacon on him. Yamagata appears to have spent that month tied to a chair with no food or water.
However in fairness, mankind in this film isn't much more intelligent either. There's a World Council, it seems, and their reaction to the news that Earth is going to be destroyed is... nothing. They ignore everything Japan's telling them and say they don't want to use their nukes. Eventually they back down and agree that it might not be a bad idea to try to save the planet, only for their mighty arsenal to have no effect at all. Sure enough, it's our Japanese hero's formula that's going to save the day after all. This film could be read as simultaneously stupid and subversive. It's peddling an anti-nuclear message and the world's conventional nuclear powers are worse than useless, but the enlightened space pacifists aren't much better either and it's super-nukes that save the day.
Note also the kidnapping of Yamagata by someone who wants to sell his formula on the international market. This film has a fat streak of cynicism.
There are two reasons why you might want to watch this film. The first is all that stuff I've been talking about. If they'd only played up all those ironies and stupidities, this film would have been brilliant. Unfortunately conventional thinking was that aliens in the 1950s were either: (a) hostile, or (b) superior beings to be obeyed in all things, so it never occurs to this film to point out that its aliens are a bunch of twonks. However manic MST3K-ing and/or obsessing over accidental subtext could perhaps transform this film's message into something interesting.
The other reason is the alien design. For the most part this is a solid production, which in its day won Asia-Pacific Film Festival awards for Sound, Cinematography and Special Effects. However the aliens were designed by a famous avant-garde artist, Taro Okamoto, whose work here is a glory of 1950s kitsch. They're starfish with one big eye, that lights up. Their skin is so obviously fabric that it would seem obvious that we're actually looking at Pyran spacesuits and there are much more convincing aliens hidden inside. These guys are awesome. Every time a walking starfish comes on screen, the film becomes a thousand times more entertaining. What's more, given the pedigree of their designer, it would seem that this is deliberate instead of just being the usual kitsch of zero-budget filmmaking. Unfortunately they don't show up very often.
None of the actors are interesting. However there are two big dance numbers.
I didn't like this film. It's stupid, but in an understated way that isn't letting you hoot at the screen. The starfish look silly, but they're not in it as much as you'd think and all other visual elements in the film are well designed, lit and photographed. This film is too solid and well-made to be obviously laughable, which is why it's dull. SF and monster movies in the 1950s tend to be unwatchable, from which you can escape either by being good or bad. This film achieves neither of those things, although you might find it in you to be entertained by the cliched finale of animals coming out into the sunlight as children sing. In many ways it's pretty good, but I was struggling to keep watching.
It's based on a novel, apparently. Amazing. I'd have said it was more likely to be based on one of the director's bad dreams or something.