Toaru Kagaku no Railgun: Arts and Sciences City SS is short story in Arts and Sciences City.
Taking place in the timeframe after Sherry Cromwell attacked Academy City, but before the events of the 'Remnant'. The cast of the Railgun series went to the Arts and Sciences City on the west coast of America 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 Railgun Cast went to the beach for some relaxation, 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.
At night, Mikoto snuck out from the hotel everyone is staying at to find out the truth of the plane that crashed. She made her way to a hangar from some hacking (and cannot believe just how easy the firewalls are outside of Academy City).