Poltergeist Arc

The Poltergeist Arc (乱雑開放) or the Poltergeist Incidents is a story arc that occurs within the Toaru Kagaku Railgun sidestory. It focuses on the series of earthquakes that are happening all around Academy City, which are called Poltergeist. It is the second anime-introduced arc and is part of the Toaru Majutsu no Index canon. The arc reintroduces the Capacity Down, and the closure to Kiyama Harumi's quest to revive her students that was first introduced in the Level Upper Arc.

Background
Kiyama Harumi, the developer of the Level Upper has been released by the Heaven Canceller using his vast influence in the city; with his help, Kiyama Harumi was able to locate all her students that were experimented on by Kihara Gensei, and to create a cure for their comatose state. She discovered a way to treat them, though her efforts have a great cost, everytime her students would try to wake up from their comatose, they would cause an AIM Diffusion Field Resonance with each other, triggering a RSPK Syndrome occurrence where ever Haruue Erii is, as she is the one who accidentally triggers the Poltergeist occurrences, as she communicates telepathically with one of the comatose children, Edasaki Banri.

Sporadic earthquakes in Academy City
Since Haruue Erii knew that her friend, Edasaki Banri - is asking for her help, she searches for her all-around Academy City. Every time Kiyama Harumi would attempt to wake Banri and the other children, they would cause an AIM Diffusion Field Resonance, and at the same time Banri would try to contact Erii telepathically, triggering a Poltergeist-earthquake.

Because of her constant search for her in various locations in Academy City, the earthquakes appear at random. Below is the list of named areas that were affected by earthquakes, before it's confirmation as a Poltergeist-related event.
 * School District 7
 * Asunaro Park
 * School Garden
 * School District 19

Academy City's response
In a meeting between Judgment, Anti-Skill and Academy City's disaster response organization, MAR, it was revealed that it was caused by an RSPK Syndrome occurrence, and not something that is related to supernatural phenomenon, as rumors are spreading around the populace of Academy City. Here, Judgment and Anti-Skill is tasked to stop the spread of rumors in Academy City and prevent mass hysteria from occurring, as to lessen collateral damage, while MAR responds to poltergeist-related occurrences, and are deployed in crowded areas.

Poltergeist incidents continue
The next large-scale Poltergeist incident occurred at the Fireworks Festival, where Misaka Mikoto, Shirai Kuroko, Saten Ruiko, Uiharu Kazari and Haruue Erii are attending. The earthquake caused massive damage, but no casualties were reported, due to the quick actions of MAR and it's leader Therestina.

Another Poltergeist incident occurred on the next day in Nature Park, where Erii and Uiharu are spending time together. The poltergeist occurrence caused massive damage to the park, and several casualties, numbering in 72 people injured, with 8 people in critical condition. Many of the victims were rushed to the nearby 5th Emergency Treatment Center for medical care, including Uiharu. Haruue Erii is transferred to the MAR Base Research Laboratory, under suspicion of causing the poltergeist incidents and for treatment.

Discovery of Kiyama Harumi's students
Misaka Mikoto investigates the abandoned Advanced Education Department, after Haruue Erii divulges her relation to one of Kiyama Harumi's students, Edasaki Banri. Here, she discovers that Kiyama Harumi is looking for data on the Crystallized Esper Essence, as she discovered that it is the key in reviving her students. She later tells to Mikoto that she is the one causing the poltergeist incidents, as she has discovered a way on waking them up with the assistance of the Heaven Canceller, and every time she tries to revive them, they cause an AIM Field Diffusion Field Resonance with Haruue Erii, causing a poltergeist incident.

Later, Therestina takes custody of the children, after following Misaka Mikoto. Despite her protests, Kiyama Harumi yields to their demands.

Therestina's deception
Uiharu visits a dispirited Kiyama Harumi, and urges her to help Therestina in waking the children up. When they reached the MAR Base, they discover that Therestina is the granddaughter of Kihara Gensei, the instigator of the experiments on Kiyama Harumi's students and the creator of the Crystallized Esper Essence. After assaulting Kiyama Harumi, Uiharu and Harumi are escorted out of the building.

After reporting of Therestina's true nature, Misaka Mikoto goes alone to confront Therestina, feeling guilty that she led her to the comatosed children. Despite her best efforts, Mikoto is defeated by Therestina in her mechanical body suit and with the help of Capacity Down, which she tells Mikoto that she invented it. After Therestina leaves her, Kongou Mitsuko (who was in the care of MAR, as she was a victim of the poltergeist incident) rescues her.

Child Error Rescue
Uiharu discovers that Therestina plans to transfer the children to a private laboratory owned by Kihara Gensei, and also discovers that Kiyama Harumi is tailing them. With the insistence of Saten Ruiko, Mikoto apologizes for going alone, and later resolves to rescue the children with them. Judgment contacts Anti-Skill on Therestina's plans, and urges them to assist them.

They later intercept an attack on Kiyama Harumi and saves her, Shirai Kuroko and Kongou Mitsuko face-off against squads of MAR, while Mikoto rides with Konori Mii who is following Kiyama Harumi's car, where Uiharu and Saten instructs her on the destination. They are later attacked by Therestina who now uses a large mechanical mobile suit; despite Therestina's efforts, Mikoto is able to destroy her suit, with the help of Shirai Kuroko (she caught up with Konori Mii, who she rides with until she arrives near Mikoto's location) who teleports one of the suits parts to Mikoto, as she uses it as a much more powerful railgun.

They arrive in an abandoned propulsion laboratory, here - they discover that all the comatosed children and Haruue Erii are safe and sound. However, Therestina returns and activates Capacity Down, incapacitating Mikoto, Kuroko and Uiharu; Saten who is unaffected by the device is hinted by Uiharu to find the control room to destroy the sound system. After severely beating Harumi and Mikoto, Therestina reveals her plans on using the children for an experiment using the Crystallized Esper Essence, to attain the coveted level 6. She is however, aware of the dangers of destroying the city when she uses the crystal on the children, but is apathetic about it. Before she begins the experiment, Saten is successful in destroying Capacity Down. Allowing Mikoto to strike back, they duel with railguns and despite Therestina's facsimile on Mikoto's railgun, she is finally defeated by her.

Kiyama Harumi, uses the crystal to revive the children, and finally is able to hear their voices again in a tearful reunion. Harumi later thanks Mikoto, where the latter blushes and smiles.

Aftermath
Therestina is later treated for her injuries, and is later taken into custody, she is later reported to be uncooperative with the interrogations. Meanwhile, the Child Error's are taken into a medical facility for rehabilitation, after years of being bed-ridden; they later prepare a message on a blimp for Kiyama Harumi as her birthday present (at the suggestion of Misaka Mikoto). Kiyama Harumi later sees the blimp from her room, with the children saying "I love you" to her, making Harumi weep.

Trivia

 * The Poltergeist arc is the only arc in the Railgun anime adaptation to have its arc concluded in the next season instead of the last episode of the previous season.
 * It is unclear whether or not the Poltergeist Arc should be considered canon. In favor of its inclusion is the existence of Haruue Erii and Therestina Kihara Lifeline elsewhere in the Index universe.
 * Haruue Erii appears in a picture on Misaka Mikoto's phone in the Toaru Kagaku no Railgun manga.
 * Haruue Erii also appears in a colored illustration in the first Shinyaku Toaru Majutsu no Index light novel.
 * Therestina Kihara Lifeline and her schemes (regarding the Ability Body Crystal) during the arc are referred to in the 22nd Toaru Majutsu no Index light novel.
 * Kihara Enshuu mentions Therestina Kihara Lifeline by name in the 4th Shinyaku Toaru Majutsu no Index light novel.
 * In favor of its exclusion is the presence of Kongou Mitsuko and the continued absence of Kiyama Harumi and her students.
 * Kongou Mitsuko does not transfer to Academy City until about August 27th in the manga, and would not have been present before August 10th when the Poltergeist Arc takes place.
 * Kiyama Harumi does not appear again in either the Index novels or Railgun manga after her defeat and arrest in the Level Upper Arc.
 * But whatever the canonical status of the Poltergeist Arc may be, the Railgun manga and anime do not appear to be consistent regarding presence (or absence) of Kongou Mitsuko in Academy City before August 10th.