Mikoto's trying to enjoy her last day of summer vacation, but she is being stalked by Mitsuki Unabara, the Tokiwadai Middle School principal's grandson who's been chasing her since last week. In an attempt to make him give up, she pretends to go on a date with Tōma. During their fake date, Tōma learns that only 10 of the Sisters remain at Academy City while the rest went overseas for treatment. Tōma converses with Unabara but is surprised to see another person who looks exactly like the latter running into a restaurant that Mikoto has entered. As the second and injured Unabara warns Mikoto about an approaching danger, the first Unabara remarks to Tōma about espers who can shift appearances, and Tōma realizes that he is conversing with an imposter when the latter tries to hit him from behind. As Tōma flees, Index reveals by phone that the imposter is using the magic of Aztec priests to disguise himself with another person's skin and wields an obsidian spearhead as a replica of the lance of Tlahuizcalpantecuhtli. When Tōma is forced to fight him, the imposter reveals that he had been sent to become close to Tōma's friends (such as Mikoto) to prevent the formation of the "Kamijō Faction", a group of mages and espers lead by Tōma which both the magic and science sides fear will destroy the truce between them. Since the imposter had fallen in love with Mikoto, he decided to protect her by killing Tōma, but, now defeated and severely injured, he asks Tōma to promise him to protect her since they'll be others like him after Tōma and his friends. Unknown to both of them, Mikoto is listening nearby and is embarrassed to hear Tōma's answer.