Toaru Kagaku no Railgun: Arts and Sciences City SS is a 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. Due to the climate apart from sleeping everyone wears their swimsuits all the time.
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, and the rivarly with Academy City. 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 and others was dragged into a mock-fight between actors dressed as pirates, and Mikoto is annoyed that they were involved without choosing to. Then they saw two strange planes being chased by modified F-22 planes. The planes being chased are oval shaped and look like a flying fish, 5m in length with four wings, and made of wood, cloth and obsidian stones. It looked like two canoes being stuck together. As everyone wonders if this is another show, Mikoto found it strange that something made of wood can travel at Mach 1 without falling apart.
Those planes are called 'Mixcoatl', meaning 'snakes of the sea of clouds'.
As one of the Mixcoatls was shot down and is about to crash onto civilians on the beach Mikoto stepped in to blocking the crash landing using the iron sands on the beach. Most people treated it as another show. As the crashed Mixcoatl struggled to get back up tourists approached it without caution, still thinking it's just another attraction. Mikoto tried to blast it back into the ocean after seeing it fired off a missile onto the tourists and moved to grab a arcade coin hidden in a secret compartment in her sandals, but it got jammed due to the sand. Mikoto blasted the missile down but the Mixcoatl then crawled over to her and tried to take her down, somehow able to walk on land like a insect. As Mikoto is pinned down by the Mixcoatl she saw the cockpit is empty and though she can't read the glyphs from how it is constantly changing and looks like decreasing she deduced it is set to self-destruct on her. Mikoto then brought the iron sands around her to form a barricade to prevent bystanders to be caught in the blast, and blasted the Mixcoatl away using it as a bullet for her railgun. The crisis is averted for the time being.
Beverly looked on and only sighed that CGs, VFXs and the such cannot match a real esper. Which proved her point back when talking to Mikoto and others that Arts and Sciences City currently only exists because Academy City has no interest in the entertainment sector.
Afterwards Saten and Uiharu tried to look for a place for dinner, and Mikoto rushed to them with tears in her eyes and a scared expression on her face. As Saten and Uiharu asked why Mikoto passed a catalogue to them, showing more swimsuits that puts what Kuroko had on earlier to shame. Kuroko was muttering to herself what should she wear next while looking at that catalogue, and Mikoto ran out from her shock. Both Saten and Uiharu regretted looking through that catalogue, seeing swimsuits that only has 5cm of cloth to cover down there, and instead of being 'T' shaped it's somehow 'I' shaped and none of them can think of just how it can stay on. After seeing someone else walking by with a sarong Mikoto decided to go forcibly wrap up Kuroko with that, then the four of them went out for dinner.
Mikoto and other met Beverly at dinner and they talked more about the Arts and Sciences City, with Saten commented once to Uiharu she suspects Beverly is at least 'G' cup, and possibly maybe even 'I' cup. Beverly commented offhand she is 'L' cup, much to the horror of the four developing girls (Beverly's bust size is possibly larger than all of their's combined). Mikoto commented even sunscreen can be used as weapons as she approached Beverly with a black aura, and after a few minutes Beverly left they realised once again they forgot to get her autograph.
After getting back from dinner, Mikoto decided to hack into the Arts and Sciences City's network to find out more about the incident with the flying fish, in case something else similar will happen. Mikoto got in and found the defenses is ridiculously easy to break through since she's used to better defences in Academy City. She found a sub-network that's not connected to the main network and decided to go check that place out personally.
Mikoto found herself at a building site 3km in diameter, named 'Large Launcher'. Here is a place to launch rockets in the film that led to this city's creation, and it was mostly left as is due to fans obessessions of wanting to keep it that way. As Mikoto picked the electronic lock and is about to go in she's found by Saten, who mistakes Mikoto as trying to get into a Casino and gamble at a legal place to do so. As Saten rushed in Mikoto tried to follow her and tell her it's not a Casino, but after Saten is convinced the electronic lock is back online and messing with it once more may lead to alarms sounding, so Mikoto had to take Saten with her to go in deeper.
After a while they found a hangar filled with remnants of the Mixcoatls retrieved from battlefields by the Arts and Sciences City. As Mikoto tried to take a photo of the name 'Mixcoatl' on the side of a plane to figure out what it is later, Olive Holiday came in and told them what it meant. Mikoto and Olive doesn't not get along well and is on the brink of open war before Olive recieved a call from the Adminstrators (Equivalent to Academy City's Board of Directors, except there is only 5 of them for the Arts and Sciences City) telling Olive to stand down. If a Level 5 of Academy City were to go missing on a feild trip Academy City will see this as a provocation and possible leak of valuable DNA maps, leading to an all out assualt on Arts and Sciences City that will include but not limited to economics, military and information warfare. Olive grudging acknowledged there's more to be lost than gained by opening hostilities and told some of her underlings to escort both Mikoto and Saten outside. If given the chance Olive will be more than willing to kill Mikoto.
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.