It's offensive, inappropriate and aimed at losers with warped ideas about this strange impossible thing called a "girlfriend". It was also investigated by Japan's Ethics and Program Improvement Organization, after complaints about its content being inappropriate for its timeslot.
Once you've braced yourself for all that, though, it's good.
The main characters are Mitsuki, Yuuya and Hiyori. Yuuya's father has just got remarried, to Mitsuki's mother. Overnight, Yuuya has thus acquired a step-sister and Mitsuki similarly a step-brother. What's more, the newlyweds have to fly off immediately to India for work, leaving their children to live together unsupervised for months. Might there be hanky-panky in the air? Not if Mitsuki's got any say in the matter. She's moody and initially reluctant even to acknowledge Yuuya's existence. Dragging a normal conversation out of her is like pulling teeth. She can also be embarrassed by perfectly normal things, with the most extreme example being a reluctance to tell Yuuya that she needs to go to the toilet. That's a girl with problems. (Bladder pain, for instance.)
Meanwhile Yuuya definitely won't be making any moves on his own sister, even one who's not related by blood. He's more worried about whether she's okay and how it's going to work house-sharing with someone like this.
So that's Mitsuki and Yuuya. I liked them. Mitsuki's a bit messed up, but she's basically a good girl and there's no reason why she couldn't learn to unbend. Besides, she's had really bad experiences with her biological father and her mother's subsequent boyfriends. Unfortunately we also have Hiyori, the third member of the triangle. Hiyori is a ghost/angel. When she was alive, she fancied the arse off Yuuya. (That's almost all she remembers. Even her current name she chose from a children's book.) Now she's dead, she needs to have all those sexy feelings and experiences she didn't have when she was alive, or else she can't rest in peace. She needs a body for that, but that's no problem. She can borrow Mitsuki's!
In other words, Mitsuki is now being intermittently possessed by a shameless sex pest with a one-track mind and the subtlety of a charging bull. They're time-sharing her body. Oh, and if Hiyori doesn't get to do all those filthy things she wants, both girls will die.
That's a hentai premise, basically. Pure-minded girl must shag her brother or die. Audience looks for brain bleach. Oh, and in case all that wasn't offensive enough, Mitsuki's being forced to wear a chastity belt. I repeat: a chastity belt. Apart from being creepy, this makes no sense given Hiyori's stated objectives. It has a "retract" button that lasts for three minutes to let you use the toilet (and then can't be used again for another hour), but that's never going to be long enough for sex. Hilariously and horrifyingly, in one episode, Hiyori is wondering if three minutes would be long enough while preparing to seduce Yuuya. She's got a lot to learn. In practice, the chastity belt's plot purpose is to be seen by other people and humiliate Mitsuki by making her look like a pervert. (She should wear opaque tights, or at least a longer skirt.)
Hiyori also likes masturbating, if that's the right term for "two people in the same body" self-pleasure with an unwilling partner. Technically I think she's doing it more to gee up Mitsuki. I don't think the English language has a word for this.
In short, this show is pretty much radioactive. The broadcast decency investigation came from complaints that 10pm was too early a timeslot, since teenagers might be watching. Personally I think that's a bit late for a TV watershed... but in this case, it's hard to argue that too strongly.
Despite all that, though, it's watchable. If you know what you're letting yourself in for, you can just accept the dodgy content and get on with the show. Underneath, it's less sleazy than it looks itself and indeed I think it's rather good.
Firstly, the show's not about what it thinks it's about. It's advertising itself as a sex comedy, yet it doesn't have any sex. Instead, it's a show about a painfully repressed girl (Mitsuki) being bludgeoned out of her comfort zone by a very, very annoying but well-meaning friend (Hiyori). That's a lot of bludgeoning. It could knock planets from their orbit. However it must be admitted that Hiyori's a far happier and better balanced person than Mitsuki. There are simple things she can teach her, such as talking to people and being able to enjoy physical activity. No, not that. I'm talking about tennis and football. It's also undeniable that Mitsuki's definition of "humiliating" is, um, stretched by having Hiyori in her life. This nearly kills her. However any girl who can't bring herself to admit to a boy that she needs the toilet is in urgent need of perspective on the meaning of embarrassment.
Mitsuki and Yuuya both have issues from their past. Neither of them is great at saying what they think, not just Mitsuki. (Ep.12 does something with Yuuya that's jarring, but interesting in how it reflects this theme.) Furthermore, despite Hiyori's worst efforts, neither of them is looking for sex. They're not completely made of stone, but so far this is a show about two people learning awkwardly about getting along with each other.
Maybe the show will stop being platonic if they make a second or third season? I don't know. Could be. There are signs towards the end that romantic feelings might become significant in future episodes, but I can only judge what I've just watched. The live-action movie looks steamier, though.
The supporting cast are in some cases underused. The main three characters are so clearly the focus of the series and their problems (e.g. Hiyori) are so vivid that the rest of the ensemble doesn't always get much time in the spotlight. I don't know why they bothered including Mitsuki's aunt Nanami, for instance. I was thinking the same of another girl too, almost until the end, until a throwaway moment suddenly made me wonder if she might be an embodiment of God or the author. Neither is impossible. After all, the angel-winged Hiyori's mission is to build a stairway to heaven. Note also a certain shot in the opening title sequence.
On the other hand, I liked Yukina a lot. I particularly liked the way she can obviously see Hiyori, but doesn't interfere and doesn't even say anything. Funniest, though, is Shoutarou, the contractually obliged Perverted Best Friend. This character type is mandatory in this kind of show. They're always annoying, but what's great about Shoutarou is that for all his nonsense about sexy sisters, he actually has a real little sister and they can't stand each other. They fight non-stop. "Don't get in the water; you'll pollute it." This is funny, but it's also grounding the show (to some extent) with a down-to-earth antidote to all that supernaturally-fuelled potential incest. (Is it incest if they're not really siblings? Well, it's still icky. Mitsuki doesn't turn sixteen until her birthday around the Christmas OVA 13th episode, incidentally.)
The show doesn't even have as much fanservice as you'd expect. The only change I noticed in the uncensored DVD edition was the chastity belt now being visible. It's not showing anything, although it still manages to look deeply dubious.
The series can get silly. I found ep.7 a bit too much for me, with those implausibly over-ambitious sports clubs and that unlikely school play. Ep.8 can get daft too. However all that's hand in glove with significant personality problems and deceptive darkness. That cute and really weird closing title sequence with a man-sized walking stuffed toy, for instance, is a reenactment of the last day of Hiyori's life until (and including) the traffic accident.
The one thing the show isn't, paradoxically, is wish-fulfilment. You'd never choose Mitsuki as a girlfriend. You'd be walking on eggshells around her. Yukina is a more obvious choice, although I note with interest Yuuya's analysis of her towards the end. Hell, even Hiyori would be better. Leaving aside her peculiarities, at least she's always cheerful, positive and full of energy. However it makes the show far more interesting to be released from being the fantasy girlfriend fodder it might well have been aiming for, if we're going to be honest. Mitsuki's a stronger character for it. We can empathise with her and feel her pain, of which she gets a lot.
The show works. I liked it. I'd even recommend it, albeit not to normal people. I wanted to watch more episodes and I think it's even marathon-able. (I also like the theme song.) All the bad taste and humiliating sex stuff works dramatically because of the effect it's having on Mitsuki. That's the whole point of the story. She gets pushed so far from her starting point that, towards the end, there's an episode where Hiyori disappears after a fight and Mitsuki gets upset and goes looking for her. Hiyori might be a walking libido, but she's also wholehearted and a devoted friend (within limits). I think in certain ways it's a genuinely good show... but with the live-action film, I'm cheerfully assuming the worst.