The Big Spider Arc is a minor story arc that occurs within the Toaru Kagaku no Railgun sidestory. It focuses on Big Spider, a faction of Skill-Out that has recently increased its attack on espers, and Konori Mii's involvement with them. This story arc is notable as it is the first arc made for the anime, however it is considered canon.
This arc also introduces the first incarnation of Capacity Down, a device that does not appear until several volumes later in the Toaru Majutsu no Index light novels.
Big Spider, a faction of Skill-Out that makes its home in The Strange, an area within District 10 of Academy City, has recently increased its attacks on espers in District 7 as a result of acquiring the Capacity Down. However, Kurozuma Wataru, the former leader of Big Spider has returned and sets his goal in disbanding Big Spider for good.
Response to Esper Attacks
Kurozuma Wataru, the founder and former leader of Big Spider has been recently released after two years in a correctional facility, and is determined to disband Big Spider. He saves Kongou Mitsuko from a small group of Big Spider members. His actions have been noticed by the current leader, Hebitani Tsuguo who now calls himself as Kurozum Wataru as a way to keep Big Spider together, he demands the current incarnation of Big Spider to take him down not knowing the true identity of their leader.
Intervention of Misaka Mikoto
Due to Konori Mii being unable to join her colleague, Misaka Mikoto instead goes along with Shirai Kuroko, to find Big Spider in The Strange. They encounter Wataru in The Strange who beats up a bunch of Skill-Outs for them, they follow him hoping to learn more and asks him about Kurozuma Wataru but gets no answer.
Big Spider's attacks on espers continue, however one attack on an esper involved Misaka Mikoto, using one of the members of Big Spider, she and Kuroko are able to locate Big Spider's hideout. They try to take them into custody but are stopped by Big Spider's use of Capacity Down, unable to use their powers, Mikoto and Kuroko are at the mercy of Big Spider. However, Wataru comes in just in time to disable Capacity Down, he then takes on a large number of armed Big Spider members and defeats them all, forcing Tsuguo to retreat. After the battle, Mii appears before Wataru, displaying familiarity with him, much to the surprise of Kuroko and Mikoto.
End of Big Spider
Konori Mii's Past
Mikoto and Kuroko being curious about why Mii is familiar with Wataru, goes to Mii's dormitory along with Uiharu Kazari and Saten Ruiko. They meet Mii's roommate, Yanagisako Aomi where she tells them Mii's past affiliation with Big Spider. Meanwhile, Konori Mii is in The Strange, she is then confronted by Misaka Mikoto on why she is here and not in the place she is supposed to be in. Their conversation is joined by Wataru who tells her that she should go back to where she belongs and that he will destroy Big Spider.
Konori Mii eventually comes to terms with her past with the help of Mikoto and Kuroko, she wears her red jacket from her days as a member of Big Spider along with her Judgment armband.
Crackdown on Big Spider
Anti-Skill initiates their raid on The Strange. Wataru arrives at the new Big Spider hideout and attacks, alongside Mikoto who destroys the Capacity Down with her Railgun and Kuroko who disables the guns of the members, Mii also steps in and disables a gunman. Hebitani Tsuguo having all his members beaten decides to use dynamite strapped to his body to destroy them, but he is quickly defeated by Wataru.
Big Spider is disbanded, as both its leaders are taken into custody by Anti-Skill. It is unknown whether or not other members managed to escape the raid.