It completely changes your idea of what kind of show it is. Twice. That's basically three different shows in twelve episodes. Not-so-coincidentally, I see it's also based on the first three books in a light novel series.
(Annoyingly I can't just call it Aria, incidentally, since that's the name of a much-praised utopian SF manga and anime that I plan to watch one day.)
ARIA THE SCARLET AMMO v.1
A successful exercise in comedic misanthropy. On top of that, there's a pleasant surprise when the show avoids the sleaze suggested by its core concept. Our hero, Kinji Toyama, is a student at Combat Mercenary School (not its real name) who gets superpowers when sexually aroused. He can knock bullets from the air. He can send his own bullets down the muzzles of his enemy's guns. He also becomes a suave ladykiller capable of delivering the world's corniest dialogue with a straight face.
You can see how an idea like that could be taken in... um, different directions. For a recent example, see Masou Gakuen HxH and its hero who can recharge girls' combat energy by having erotic experiences with them. (Or, alternatively, don't.)
This show, though, subverts that. Firstly, Kinji hates his Hysteria Mode (as he calls it). He wants to quit this school, his principles are based on being a selfish bastard and he finds his suave alter ego embarrassing. (Hysteria Mode has other downsides too, since Hysteria-Kinji can't hurt a woman or even refuse any of their requests. Since almost all of this show's supporting cast is going to be female, including its villains, this will pose problems.) As a result, Kinji goes out of his way to be obnoxious to girls and will actively avoid the potential annoyance of, say, seeing their underwear.
Secondly, the show itself stays clean. You won't see Aria's knickers even when the camera's pointing straight down at her mini-skirt as she hangs upside-down from a speeding bus. She's also violently opposed to even the idea of romance. She's a tiny, bullying tsundere who calls Kinji her slave, bickers with him viciously and basically treats him as he deserves. The Kinji-Aria mutual abuse is funny and I enjoyed it a lot. She's glued herself to him, mind you. Termites are easier to get rid of. No matter how obnoxious he is to her, she doesn't go away and just responds in kind, often filling the air with bullets.
Of course they calm down a bit. They even spend some date-like time together in ep.3. However they're both vehemently anti-romance and the storyline's mostly about some villain who must love Speed (the 1994 Keanu Reeves movie) because it gets done twice. The only person who sees potential romance there is Kinji's childhood friend, Shirayuki, who has a bit of a yandere thing going on. She'd be the model of traditional Japanese femininity if it weren't for the occasional purple aura of death as she tries to kill a perceived rival for Kinji's non-existent affections.
It's fun. The story's hardly deep and not without idiocy, e.g. don't stick your head out of cover when Uzis are being fired in your direction. However this is a "guns can't hurt our heroes" show (semi-justified by bulletproof school uniforms) and in any case you don't expect realism from a show that begins with a girl parachuting from a building in a mini-skirt in order to shoot robots. I had a laugh and I liked the Kinji-Aria combination, even if I probably shouldn't. They're pretty appalling human beings, but they're entertaining light novel protagonists.
ARIA THE SCARLET AMMO v.2
Ep.4 turns everything upside-down, twice. Our heroes get backstories and emotional justifications for their behaviour. I was taken aback. Character depth? Motivation? I'd already been enjoying the show, but this improved things considerably.
And then we have some revelations so awesome that I can't not discuss them. They're massive spoilers. I'll go into them at the review, after lots and lots of spoiler space. Suffice to say that realism is now even less of a consideration.
This was my favourite phase of the show and I was having a whale of a time, although I did think Kinji's Hysteria Mode could be a bit too convenient for the plot. Making him less powerful might have been good for the storytelling.
ARIA THE SCARLET AMMO v.3
...and, oh dear, it's a light novel series after all. Hello, harem. Kinji's a male protagonist surrounded by girls, of whom two are capable of trying to have sex with him in broad daylight in front of everyone. (One of them's probably mad. The other's just shameless and she suggests a threesome.) The show also starts indulging in fanservice, with girls stripping to their underwear, or further (in the OVA episode). Technically the show generally maintains its rule of "no upskirt shots", but that only applies for girls who were wearing skirts in the first place. Reki doesn't count in ep.10, for instance, since the only thing (not) protecting her modesty is Kinji's coat.
The show's still a blast, mind you. I had fun. It even manages to do interesting things with the harem genre, since Kinji is an unusual harem hero and for once has a reason beyond genre contrivance for keeping his distance. However I was also a bit disappointed in it and hence less inclined to overlook its occasional failures, e.g. some lack of clarity in the action in ep.12, or bullets being useless even when your targets have no bulletproof clothes at all (because everyone's naked in the hot spring).
They're also setting up a Season 2 that never happened (e.g. Kana), although there's also been a 2015 anime adaptation of its spin-off manga, Aria the Scarlet Ammo AA. I'm looking forward to that enormously, of course. In fairness, too, this anime only adapted the first three books of a light novel series that's currently at 21 volumes and counting.
I liked this show, then I loved it, then I merely liked it again. It's a shame it didn't get a Season 2, since I think there's a lot to like even given the harem angle and apparently there's a lot of good material in the light novels. The show has its bashers, mind you. It wouldn't be hard to take a dislike to any of the regulars. Aria in particular is yet another pint-sized tyrannical tsundere romantic lead played by Rie Kugimiya, as is often pointed out by those who dislike the character. Personally I like her, though. I think she's the perfect match for Kinji and for me the show does remarkably well at engaging me with the adventures of these potentially unlikeable characters. (The line between hero and villain can be very blurry.) As for the setting... well, no, it starts out unrealistic and then heads further into fantasy. I like that too. That's simply its genre.
It's funny. It has emotional depth. It has outrageous action. Recommended! Now for the spoilers!
Everyone's got a famous ancestor, although unfortunately they're all defective when it comes to being a genius. (That's not me being snarky. It's canonical. The characters themselves comment on it.) Aria's great-grandfather was Sherlock Holmes. (Holmes had offspring? Really? Goodness me.) I then remembered the opening title sequence and howled. Aria can have impressive intuition and she speaks seventeen languages, but her main point of similarity with Sherlock is being insufferable.
What makes this really cool is her enemy, Riko Mine Lupin IV, being the great-granddaughter of Arsene Lupin. (Maurice Leblanc really did write Lupin-Holmes crossovers, by the way, although he changed the name to 'Herlock Sholmes' after legal objections from Conan Doyle.) I loved this. It's acknowledging both the original Leblanc stories and the arguably more famous Lupin III anime franchise, which is particularly pleasing since Riko's an orphan and so at least in this universe we have the happy thought of a long-dead Lupin and Fujiko. It's surprising that Fujiko's daughter isn't the one with the biggest boobs, but she's certainly inherited a lot of her mother's personality.
Mind you, it's clearly wrong when the French Riko calls Aria a "limey". That should be "rosbif", but I imagine most Japanese audiences wouldn't have known that.
Anyway, I went crazy about Holmes vs. Lupin. It tickled me pink and also explained some character points in that satisfying way that makes you slap your forehead and go "of course"... and then things started getting silly. We meet Joan of Arc XXX (i.e. the 30th, not a porn version). Yes, Joan had children. It's a 15th century conspiracy theory. However if you do the maths, our 21st century Joan has less than a billionth of her famous ancestor's DNA, which probably puts her on a par with most of France.
There's also a scary and very old person called Vlad, which made me realise that "Vlad" and "blood" would be spelled the same if transliterated into Japanese.
As with much of this show, everyone having a famous ancestor is arrant nonsense even before you get to the particularly blatant points of silliness... but it's also cool. You could complain about things getting ever-more fantastical as the season reaches its finale, but then again it's not as if the show was aiming for realism with "great-granddaughter of Sherlock Holmes". I'd have loved it even more had Aria been really clever and/or shown detective ability, though...