Mental Out Arc

The Mental Out Arc is a story arc introduced in Shinyaku Toaru Majutsu no Index, encompassing the content of the 11th Shinyaku Toaru Majutsu no Index light novel volume. It primary deals with Shokuhou Misaki, the 5th-ranked Level 5, having to deal with a conspiracy against her that is connected to her past with Kamijou Touma.

Background
The conflict of the arc is rooted in the similarities between Shokuhou Misaki and Mitsuari Ayu, both having similar powers and both having similar memories with Touma. In accordance with Parameter List, Misaki was given focus and development for her powers while Ayu was kept as a mere back-up in the event of her death.

Ayu had already met Touma prior to him meeting Misaki. Due to the circumstances surrounding development, Ayu falls into despair and considered suicide. When she was about to go through with it, she sent a message to Touma, hoping that he would save her. However due to bumping into Shokuhou Misaki at an intersection, Touma lost his phone and as such was unable to reach her in time.

She survives her attempt at Ground Geo, however, and was then drawn to Academy City's dark side. She discovers Misaki's relationship with Touma and thus resents her for not only stealing her chance to become Level 5, but also for being saved when she wasn't. Shundou Toshizou would later make use of her, manipulating Ayu without her noticing, for the purpose of finally stealing Mental Out for himself. For this purpose, he likely lends her the equipment that she would use for her vengeance. Using the equipment, she camouflages Ground Geo using the FIVE_Over OS and somehow inserts a Strobila on Misaki in order to torment her and create doubt that would eventually lead to her to erase her own memories of Touma believing them to be fake.

Chronology
Early in the arc, Misaki reminisces of her time with Touma in the summer a year ago before he lost his memories of her. She visits places around Academy City that is significant to her memories of Touma until at last she arrives at Ground Geo. There, she falls for Ayu's ruse and believes her memories have been manipulated.

Misaki later concludes that the only other person she knows that is capable of matching her is Kumokawa Seria. She later travels to where she believes Seria is hiding. When at last she comes upon the place is caught in her trap by her. Misaki pleads her case to Seria and tells her of its involvement with her memories of Touma. Convinced, Seria helps her and soon discovers that a Strobila device has been implanted into her. Misaki later leaves with the Strobila in Seria's possession in order to find out who the true culprit behind it being implanted into Misaki, pained by the thought that her memories with Touma might be false.

However, Seria is not the only one who is aware of Misaki's investigation. Misaki is visited again by the Ringlet curl girl asking her if she needs assistance, though Misaki dismisses her again. Much more pressing however is Ayu discovering that Misaki has finally discovered what was going on. After the removal of the Strobila, Ayu had several of her subordinates approach Misaki disguised as Emergency Aid workers to replace it under the guise of treatment for instability. But Misaki sees through their ruse and manages to take control of them. She questions one of them and discovers that the Strobila had no actual data and that it was only used to make her doubt her own memories so she would erase them herself.

She is enraptured by the revelation that her memories may be real after all. She then calls upon the Ringlet curl girl and the rest of her clique that is willing to follow her as she needs their aid in finding out the truth behind the doubt in her memories, which is in Ground Geo.