When is it acceptable for a series to deviate from an established, successful formula? When the franchise's most compelling storyline and some of the franchise's most high-powered esper-on-esper battles to date are involved. Such is the case in the first half of Railgun S, which starts out with much the same approach and temperament as its predecessor but gradually narrows its focus to Mikoto alone as the story plows deep into the franchise's Sisters arc. While those wishing that the series would move on with fresher stories may find themselves waiting impatiently for this arc to play out, those who always felt that the original treatment of the story back in episodes 10-13 of the first season of A Certain Magical Index left a lot of holes to be filled should be plenty satisfied by the vastly more thorough treatment here. Those who have not seen the original treatment in Index should be able to make sense of this just fine, as this is the full-story version.

The Index version of the Sisters arc focused on the elements of the story most directly relevant to Touma, and as viewers see here, that was only a brief summary of the full picture. The groundwork for the arc gradually starts getting laid beginning in episode 2 and steadily ramps up from there, with a MISAKA clone appearing for the first time in episode 3. The rest of the series' first half delves into how the clones are created, how Mikoto first becomes aware of what is really going on with her clones, and the actions (alluded to in Index) that she takes to try to stop the Level 6 Shift project prior to Touma getting involved, including her fateful first meeting with Accelerator (also alluded to in Index). Even once Touma finally steps into the picture in episode 11 (whose lead-off vending machine scene equates to a like scene in episode 10 of Index), this series continues to show the content not seen in Index's version and explain certain things always left vague or as discrepancies about how the original version played out, such as how MISAKA 10031 fits into the picture when Touma was always interacting with MISAKA 10032, Mikoto's gut-wrenching discussion with her “little sister” after Touma meets her, or how she knows about Tree Diagram's fate. The first half ends by bringing the story to the point where MISAKA 10031 goes to fight Accelerator, along the way revising in small ways some of the scenes originally seen in Index.

But the excitement of this season is hardly just about filling in the blanks. Over the course of these 12 episodes Mikoto also comes into contact with no less than three of the other six Level 5 espers in the city. One – the mind-controller Misaki Shokuhou – appears in the first episode and shows off her power, but their conflict has yet to lead to a fight and she, in fact, has yet to even reappear since then. Accelerator pops up a few episodes later to show precisely how Mikoto discovered that she could not beat him head-on (as mentioned in Index). Later still Meltdowner, the Level 5 who leads ITEM, confronts Mikoto for a knock-down, drag-out fight which delivers a heavy dose of the kind of super-powered fight scenes that have always been a staple element of the franchise.

What these episodes do not offer, however, is much of Uhiaru, Saten, or even Kuroko once the Sister arc takes over. Past episode 4 they have only minimal appearances, including not appearing at all in certain episodes. Given that the interactions between the girls, and the camaraderie which develops out of that, was a highlight of the first series, this would seem to be a very damaging turn of events, but that is so thoroughly balanced out by the deeply compelling nature of what is transpiring with Mikoto and her clones that their absence can be forgiven in this case. The humor seen in the first series is still present here, especially in Mikoto's interactions with the first clone she meets, but so is the dark, ugly side of Academy City hinted at in the first series, which is brought into such chilling detail here that it stands in stark contrast to the humor which comes before it.

Director Tatsuyuki Nagai and his principle J.C. Staff crew are back for another round of artistic efforts, with similar results to the first series: mostly solid, with moments both spectacular and substandard. The spectacular moments are, naturally, in the action scenes, with Mikoto getting plenty of opportunities to show off the full range of her powers, whether it involves scaling walls, keeping a destroyed metal staircase from collapsing and walking up it, frying computer systems, or tossing dolls imbedded with metal around as weapons, and some of her foes get to do the same; what Misaki can do is actually rather scary even though Mikoto's powers effectively make her immune to it, Meltdowner has some impressive offensive power, and even the girl with the exploding dolls is a satisfyingly nasty threat. (And Accelerator? If you did not already know how bad-ass he is, how handily he overwhelms Mikoto will make that point clear.) The animation is at its best in these action sequences, though one of the most spectacular scenes is Kuroko and Mikoto's mid-air pursuit of a wayward helicopter in episode one, a sequence so slick that it makes one yearn to see more of these two in action as a team. The artistry is most consistently sharp in close-ups, with some stumbles in quality control in other places and descents into caricatures in places. Continuing character designs are just as appealing as before, but new characters fail to stand out as being particularly distinctive-looking, with the “crazed girl” look in particular being overdone. The animation offers a surprising amount of background detail concerning moving pedestrians and rarely resorts to shortcuts. The graphic content is decidedly stronger than in the first series, however, with some scenes involving the clones graphic enough to have been censored for broadcast and minor doses of fan service.

As with the first series, the musical score provides a respectable synthesizer-heavy mix of playful, dramatic, and darkly moody sounds but does not distinguish itself much. Opener “Sister's Noise” is a hyped-up, synthesizer-driven number which seems determined to lend its energy and enthusiasm to the series, while the moodier light rock sound of “eternal reality” closes out all but episodes 1 and 11. The entire principal cast from the first series also returns to reprise their roles, with no drop-off; Kuroko even sounds slightly less annoying this time around.

Due to its reliance on the Sisters arc, Railgun S has, so far, generally played out at a level equal to the better content from the first series. Mikoto's struggles to deal with the moral and emotional impact of the cloning affair, alongside her more physical and mental fight against the Level 6 Shift project, may not leave much room for her more amusingly childish side to show but they do provide a greater and deeper opportunity for character development and deliver a degree of pathos and desperation not as successfully achieved previously in the franchise. With some leads already established about what direction the series might go in after the Sisters arc is finished, and more integration with the Index version still upcoming, the future looking promising for this one.