The word "railgun" might be in the title, but that doesn't mean that To Aru Kagaku no Railgun has to focus primarily on Mikoto all the time - Thus, this episode turns our attentions to the history between Shirai and Uihara.
This flashback takes place against an introduction where the two Judgement members find themselves at the scene of a car being broken into - Rather than waiting for backup as suggested by Uihara, Kuroko charges in without a second thought and ends up both letting the thieves escape while also putting both girls in danger. This in turn leads to a blow-out argument between the two, with Uihara (who expected her to get angry?) losing both her rag and her patience with Kuroko, particularly when the latter seems to have forgotten a promise that the pair made in the past.
This leads us nicely into the story of exactly how that promise was made, taking us back to a time when both Shirai and Uihara were at elementary school. When a robbery commences at the post office where both these characters are situated at the time (Academy City seems to be rife with crime, doesn't it?), once again Kuroko rushes into action without considering the potential dangers of doing so, endangering her friend's life and having to rely on some mysterious outside help to help her before she eventually triumphs over the robbers.
I've got to say, this was perhaps the most enjoyable episode of To Aru Kagaku no Railgun so far - For all of its lack of everyone's favourite railgun apart from some brief appearances, it was a well-paced and nicely told story that gave us a little more depth on both Shirai Kuroko and Kazari Uihara's characters, fleshing them out in a way that was arguably required to bring them forward from roles simply as "girls who aren't Mikoto" into interesting individuals in their own right. Stories that flash back to the past can occasionally be tired, plodding or just plain unnecessary, but what we saw here managed to avoid all of those pitfalls to deliver a genuinely decent story. While I rather enjoyed the fun "eyebrow drawing" story and even the more slice of life-esque opening two episodes, this is more like the kind of thing I was expecting from this series - My only concern is that it all still seems a little disjointed, taking us from what looked to be important scenes regarding Mikoto last episode to this. Still, even that isn't enough to dampen my enjoyment of this instalment.