According to various sources, this movie is notable only for being the first Japanese science fiction film shot in color. (Wikipedia remarks that it may have inspired Starro from DC comics, but this is probably a teeny bit questionable.)
There's no way around it; this isn't a particularly good movie. Not to say that you can't be entertained by it...
It borrows heavily from The Day the Earth Stood Still (a reminder on my opinion of that film) and When Worlds Collide (sadly, I've never seen it). The plot: Peaceful aliens that look like bipedal cyclops starfish made of cloth (called Pairans, but I don't know where you would learn this from the film) come to warn Earth of "Planet R," a planet "from another galaxy" which is hurtling towards Earth and threatens to destroy it. Why do the aliens care? Because their planet is in the same orbit as Earth on the other side of the Sun, and it would suffer if something bad happened to Earth. (Roll with it, it's old and Japanese.)
Comparisons with the films that probably inspired it will generally talk about how weak it is by comparison. While I'll admit that it's no masterpiece, the benevolence of these aliens is actually much less questionable than that of the aliens from TDtESS: While those aliens pretty much just said "Disarm or die," these aliens said "We'll try kinda hard to help you."
I say "kinda hard," because the film's narrative is horribly disjointed and full of wacky plotholes. For instance, there's a scientist who knows a formula that could create a substance that would allow for the creation of a bomb "many times more powerful than the A- or H-bomb." (That is exactly how they put it, every single time.) The aliens know this formula, having developed it in their ancient past, but discarded it because they had no need for it. ("We do not recognize your trait called aggression." Paraphrased significantly.) The aliens, despite being able to recognize the formula on sight, don't actually know it. What. How. Sometimes why.
Anyway, the movie couldn't have held together if they hadn't done it that way, but it still creates a big problem: When the scientist gets kidnapped by the mob because the existence of the formula was accidentally publicized, he gets tied up for interrogation purposes, and then he gets left in an apartment building alone for a month, while the Earth heats up from the presence of Planet R (roll with it, it's old and Japanese) to the point where it's killing stupid animals. (A dog shown reacting to Planet R in the sky was later shown wandering through the street and collapsing; a cat shown sitting on a rooftop also reacting was never shown again, implying it knew better than to stay where it was hot. Score one for cats.) So the aliens rescue him after the building he's in starts inexplicably falling apart. It's then that we find out they could have found him at any time, because they put a tracking device ring on him. They get the formula from him, and then throw him back out into the street to find shelter. (Holy cheese, man, they aren't very concerned with his health, are they?)
Then, they launch a missile to blow up Planet R. And because it's old and Japanese, this doesn't cause mass destruction on Earth's surface.
One of the other interesting things about the film is that the most prominently featured alien, apparently a high-ranking individual, is female. (Or, for most of the film, a human woman.) This is probably the most entertaining part of the film, actually-the alien lady (who modelled herself after some kind of Japanese pop star to explain why they picked out an attractive actress) runs around doing super insane stuff like ten-feet-straight-up jumps, phasing through solid objects, and knowing advanced scientific formulas (forgive them if that sounds sexist, it's old and Japanese). And they have to do tests on her (or rather, on her hat and tennis racket) before they decide for sure there's something strange going on, and even then there's one guy who is all "Well, I don't know, maybe..."
I watched a few other cheesy science fiction movies of dubious origin recently, so a few more of these are coming down the line. This might have been longer if I wasn't kinda sick right now.