Toaru Kagaku no Railgun: Arts and Sciences City SS is short story in Arts and Sciences City that takes place in the time period between September 3rd to 10th. Academy City have a exchange program with the rest of the world that allows some students to travel for this week to other places, and some students are allowed into Academy City during this time.
Mikoto, Kuroko, Saten and Uiharu went to the Arts and Sciences City, made on a artificial island off the coast of California for a field trip. The Arts and Sciences City is known as a city that has cutting edge technology (in terms of the standards of the 'Outside' of Academy City - ie it is still far behind that of Academy City's), and is trying to take over Hollywood as the entertainment capital of the world. Stuntsmen and shows occur frequently at this city, and anything out of the ordinary is treated as another show.
As the Mikoto, Kuroko, Saten and Uiharu went to the beach for some relaxation, they talk about their swimsuits. When Kuroko appeared with her skin-coloured swimsuit that covers only the bare necessities the rest of them watch on dumbfounded. Even the bystanders are put off by Kuroko's appearance as her generous show of skin is more of a horror show than a fanservice.
Soon afterwards they meet Beverly Seethrough and they talk a bit about how the Arts and Sciences City was established. Only after Beverly left did they realise she's a famous director, and Mikoto is agonizing that she forgot to get a autograph (though the four of them were too busy lamenting the difference in bust size between them to care about that earlier).
Then Mikoto found a odd-looking plane that is about to crash onto civilians on the beach, and did not seem like it is just another show. She stopped the plane from crashing by using her ability, and due to most people treating it as another show instead of Mikoto is really a esper from Academy City.
Xochitl is trying to infiltrate into the Arts and Sciences City, and she was found by Saten while contacting one of her associates. Xochitl threatened Saten into helping her infiltrating if she wants to live after overhearing them (really just a ruse since most professional magicians tend not to involve normal people if possible, and Xochitl has her own professional pride), and went and told Saten to go buy a swimsuit that will allow her to blend in. Saten ends up buying a excessively revealing swimsuit for Xochitl and while it suited Xochitl perfectly in looks it is almost the very opposite in terms of 'blending in' as it is very distinctive. Saten did that on purpose and enjoyed Xochitl's annoyed look. In the end it turned out the Arts and Sciences City has obtained a crash-landed aircraft from the Aztec magicians, and plans on reverse-engineering it to gain access to its powers and set up a rival organisation against Academy City. The Aztecs seeks to retrieve their aircraft and launched an invasion using flying fortresses and a air force of their own, disregarding normal civilians' lives. Xochitl decided to go against their Tecpatl's (the leader for this operation) orders, and helps citizens evacuate instead of killing them all to prevent knowledge leaking out. This results in her punishment and lead to her conflict with Etzali in Index Vol15.
Mikoto went head to head with the full might of the Aztec airforce using her electricity powers to strip hydrogen from water and using it as propulsion. She uses those hydrogen wings to fly, but this has nothing to do with 'awakening' such as that of Kakine Teitoku's or Accelerator's. Kuroko helps Mikoto fight by holding onto Miko as she flies and teleports the two of them to evade attacks Mikoto's wings cannot dodge. Xochitl saw Mikoto's powers and is stunned by her prowress, as to her defeating an entire army almost single-handedly is supposed to be unheard of in the events of the Magic Side. How much Xochitl knows of people such as 'Saints' is debatable.
The novel concludes with Arts and Sciences City being disbanded, the Aztecs failed to retrieve their aircraft (it was destroyed in the fight) and razed the city to the ground (with little or none civilian lives lost due to Xochitl's interference), and a promise between Saten and Xochitl to see each other again someday.