I'd had this on my radar for years. It's pretty famous, despite being a little OVA series that's based on a similarly brief manga. (It ran for three volumes in 1998-1999, with a spin-off fourth volume in 2003.) What made it famous, I think, was its clash of style and content. It looks like a show made for little girls, with cartoonish faces, jolly music and twelve-year-old heroines who are still in elementary school. It looks charming. Our heroines have been voted by their classmates to represent everyone else and do an everyday school job for a year. It's got some of the tone and pacing of a slice-of-life show.
The twist, though, is that this "everyday job" involves wearing a talking winged frog on their heads and defending the school from aliens. (I called it "everyday" because it might happen every day.) The frogs feed off your sweat and grow drill-tentacles that can punch through walls and kill almost anything.
Looking back today, what surprised me is that the show's less dark than I'd expected. This kind of thing's almost a genre by now. Puella Magi Madoka Magica and its successors are playing in similar territory, but Alien Nine isn't interested in darkness for darkness's sake. It's not all about the journey to death, ultra-violence, despair, etc. The show isn't a killing jar. It doesn't even really have a plot, or at least not an ending. Instead it's an unusual sort of slice-of-life show, or more precisely a coming-of-age one. Listen to the theme song lyrics. Our heroine is Yuri Otani and she's going to spend four episodes being disgusted, confused and scared by changes in her body, unwanted hair growth, being pursued by boys, weird dreams she doesn't understand and other such things that to me suggested a puberty metaphor.
(Mind you, other people have suggested that it's about recovering from sexual abuse, or even an indictment of modern society's effects on the innocent. I disagree, but I'm sure it would be easy to find all kinds of personal resonances in the show's matter-of-fact presentation of such a surreal scenario. Also, admittedly, ep.4 is a worse fit for my suggested puberty reading than were eps.1-3. To me if feels as if that's where the story started taking a life of its own, instead of just being a metaphor. You could also choose to see incest hints in there, if you take a certain view of a certain character.)
I'm not insisting that my suggested reading is the only option, of course. However it made sense for me and seemed to tie in with lots of different angles. It fits the juxtaposition of childish style and adult content. (That fairly straightforward contrast seems to have surprised a lot of people.) It fits with ep.1's physical examination and the occasional bit of nudity, e.g. frogs feeding on your unclad back. It fits with Otani's confusing dreams, which are liable to involve boys, other girls flying away from her and/or standing naked and miserable in a blood-red swimming pool. "Everyone becomes an alien. Our bodies change." You grow scary hair and have accidents that leave everything gross and bloody. The underside of an alien frog hat looks a bit like an orifice. Otani thinks she's the only one the boys go after, whereas her friends are either confident in (Kumi Kawamura) or actively relishing (Kasumi Tomine) their alien-fighting roles. (The one who's most into it, by the way, is the one with ultra-permissive parents who never stop her from doing anything.)
One could even speculate about Otani's nascent sexuality. Being a lesbian would certainly cast another light on her relationships, but the evidence for that's a bit speculative and to be honest I almost think it would weaken what the story's saying to drag sexuality into it. She's twelve. The best argument for that angle might be her name (Yuri), actually. Alternatively you could also choose to see certain developments as a metaphor for rape rather than more simply for boys similarly entering puberty, which I think is debatable but not completely absurd given Otani's highly unusual nature as a protagonist.
Viewers who took this as nothing more than an alien-fighting story have been known to call her a useless heroine. She's miserable about everything and spends most of the show scared and crying. Personally, though, I'd think that's understandable even without metaphorical layers. It's a plausible reaction for a twelve-year-old girl who didn't volunteer for any of this and is routinely getting sent on life-or-death missions to hunt gross monsters.
Something's going on with the adults. Secrets are clearly waiting to be uncovered there. "I hadn't requested anything like this!" It's not clear that the teachers even expect a 100% survival rate, but it's all too allusive for anything more than guesses.
Incidentally, the alien frogs are called Borg in the subtitles, but I don't think that's a Star Trek reference. (Star Trek's Borg have a different Japanese transliteration.) Instead I think it more suggests the Japanese word "bougu", c.f. guard, protector, defensive weapon, etc.
I'm also not sure about all the voice acting. Every so often I'd wonder if they'd cast actual child actors. Looking up the cast... maybe they did? I'm not sure. The youngest ages I can verify are only seventeen or eighteen, though.
It's an intriguing show, but probably the most unique thing about it is the way it's undercutting its own extreme content. The art style is cute, while the narrative's making weird alien-hunting missions seem almost mundane. The important thing about our heroines' alien lacrosse hunts (no, really) is how unhappy Otani is about going on them. She's the show's focus, not the extraterrestrials. Why do they keep landing? Why are the girls told to catch the aliens alive? We can speculate, but no more. I considered buying the manga to find out, but apparently that doesn't have a definite ending either (although I believe it goes significantly further down the road of darkness). These OVAs, though, are simply exploring the intersection of bizarre violent aliens and a twelve-year-old girl's everyday life, with the former becoming simply a very large wrinkle in the latter.
"If we don't hurry up and catch the fifth one, we'll have to stay after school."
It's interesting, but to be honest I don't expect to feel the need to watch it again. I considered buying the manga since it's only three volumes, but I probably won't. I'm glad I saw it, though. In its gentle-but-horrible way, it's special.