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 T?ma. During their fake date, T?ma learns that only 10 of the Sisters remain at Academy City while the rest went overseas for treatment. T?ma 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 T?ma about espers who can shift appearances, and T?ma realizes that he is conversing with an imposter when the latter tries to hit him from behind. As T?ma 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 T?ma is forced to fight him, the imposter reveals that he had been sent to become close to T?ma's friends (such as Mikoto) to prevent the formation of the "Kamij? Faction", a group of mages and espers lead by T?ma 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 T?ma, but, now defeated and severely injured, he asks T?ma to promise him to protect her since they'll be others like him after T?ma and his friends. Unknown to both of them, Mikoto is listening nearby and is embarrassed to hear T?ma's answer.
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