It's where RAILGUN gets serious. Season 1 had two dramatic story arcs, but also lots of slice-of-life. (Often involving Shirai and Uiharu's police work with Judgement, yes, but it still feels like slice-of-life.)

Here, though, they've reached the Sisters arc. That was only four episodes in INDEX, but that was Misaka's story and she's the title character of RAILGUN. You couldn't cover something that huge and evil in only four episodes here. Here it takes sixteen episodes, which strikes me as about right. Giving a spoiler-free summary of this would be difficult, but suffice to say that we're looking at a five-figure body count and that Misaka will end up suicidal. (She doesn't do it, but only thanks to Touma. What's more, her reasons make sense and the decision's actually rational.) That's the most powerful story arc of the whole franchise. I can't see anything else topping it, if only for scale and staggeringly evil ideas. However the show then continues with the Silent Party arc, which as it happens is also pretty good. The finale manages to make half the cast seem like the coolest person in the world, at least briefly when it's their turn to shine.

So, what is this show? It's a side-story series, set during the second half of INDEX Season 1. Misaka, Shirai, Uiharu and Saten go to school in Academy City (where 80% of the students have superpowers), hang out together and get used for clone experiments that count as a crime against humanity. (Okay, that last one's only Misaka.)

Oh, and there are also Erii Haruue and Banri Edasaki. Who are they? You might well ask. I've watched all of INDEX and RAILGUN back to back from the beginning and even I couldn't remember. I had to google them, which I think is the gold standard of cast overload. I don't think the headcount's the problem. I think it's specifically Haruue and Edasaki, who were likeable-but-bland victim characters from a Season 1 filler arc and have no discernable personality traits beyond "nice". The important thing about them in their original story arc was that they're telepaths, but here you won't even notice them using their telepathy. Banri's in hospital and everyone visits her. It's pleasant to watch their scenes, but it's also generic, with a slice of forgettable on top.

Haruue and Edasaki are an argument for watching this franchise out of broadcast order. That wasn't what the production team intended, but that's your only hope for remembering who they are. I'd also note that INDEX II and RAILGUN I can both get a bit one-note sometimes (respectively with magical/religious nonsense and schoolgirl filler arcs), so mixing up the shows could add variety. Unfortunately there's a complication. You'd be watching the Sisters arc twice, but my guess (although I am indeed guessing) is that you'd probably be okay. The story's strong enough to sustain it, especially since each version is showing us a different protagonist's viewpoint. They're complementary. (For what it's worth, I compared the vending machine scene in INDEX I ep.10 and RAILGUN S ep.11 and they're very different.) If you decide to try that, I believe the chronological ordering would be:

...with INDEX II, RAILGUN III and more to come later. Mind you, we don't know where ACCELERATOR goes yet, so that might go there too when that comes out.

THINGS I PARTICULARLY LIKED ABOUT THIS SEASON (in descending order)

1. The Sisters, who blow me away. They're deadpan and emotionless, but often also funny. They'll uncomplainingly accept the most chilling thing in the world, then surprise you with their attitude to it in ep.16. "Downsizing." "Unemployed." No amount of Sisters screen time is ever enough.

2. Kuroko Shirai, who's nearly as amazing (but in this season is quite often sidelined). This voice role won Satomi Arai a best supporting actress trophy. Shirai's a sleazy deranged sex pest, a tough-as-nails Judgement agent and one of the best friends you could have. She's absolutely devoted to Misaka, although she'll need electrocuting every now and then to keep her in line. She normally talks like a slightly dessicated old lady, but she's also capable of sounding girly, sultry or flat-out bestial. What the hell are those noises? I don't know, but she's hilarious.

(Mind you, I'm getting suspicious about the show's attitude towards her powers. Do they have hidden limitations we don't know about, or else does she just get tired? If her feet hurt from too much walking, why didn't she teleport? If her friends tie her up, why doesn't she teleport out of her ropes?)

3. The action scenes. There aren't as many of them as you'd think, since this isn't an action-driven show, but at their best you'll want to stand up and cheer. Misaka in ep.1, or pretty much everyone in ep.24.

4. The story arc about Misaka keeping everything secret from her friends and trying to handle it all herself. (That's all the way through the Sisters arc, anyway. She then gets challenged on her secretiveness and changes her ways... but you'll notice that she only fills everyone in on current events, not the more traumatic recent ones.) The power of friendship is one of the most common anime themes, but you don't often see both sides of it explored like this.

5. Lots of other stuff.

6. That bit in ep.12 where two girls try to feed a cat bread. What the hell? (Some cats will eat bread and it's actually okay for them, but only in small amounts since it's so different from their natural diet.)

7. Eating rusty nails.

8. Kinuhata Saiai of ITEM and her chou-annoying chou-verbal chou-tic.

This season is probably the best of the five instalments I've seen so far from this franchise. In descending order, the others after that would go INDEX 1, RAILGUN 1, INDEX 2, MOVIE, with the "stop watching here" point coming between INDEX 2 and MOVIE. (INDEX 2 doesn't always clear that hurdle, but as a whole it's okay. MOVIE isn't.) That's only up to 2013, though, so I'm looking forward to the 2018-2019 revivals. This season's problems are just a couple of unmemorable characters, really, which isn't much. Admittedly even this "best of the franchise so far" season isn't a breakout must-watch show, but it's a solid eight out of ten that always entertained me.