"July 24, Part 4" (七月二十四日④ Shichi-gatsu Ni-jū-yokka ④?) is the fourteenth chapter of the Toaru Kagaku no Railgun manga series, released on May 21, 2008.
In a flashback, Kiyama Harumi shouts at Kihara Gensei about the outcome of an experiment gone wrong. Gensei replies that this outcome is good since they are getting rid of the Child Errors.
Back to the present day, Harumi looks at an apparently unconscious Misaka Mikoto, who is buried under the rubble she created. She turns around, only for Mikoto to grab her from behind. Harumi is confused as Mikoto shouldn't have been able to avoid her surprise attack. Mikoto explains that her body constantly emits electromagnetic waves that allow her to sense her surroundings. Harumi tries to attack Mikoto with metal cables but Mikoto shocks her first. As Harumi collapses, Mikoto starts seeing Harumi's memories due to the electric shock linking their minds together.
In the past, Gensei ordered Harumi to teach children, something she was reluctant to do as she hated children. Gensei convinced her by saying that the children were Child Errors and thus important for research. Over time, Harumi formed a bond with the children, in particular Edasaki Banri. Eventually, the children were subjected to an experiment, with Banri saying that she wasn't scared since she believed in her teacher. However, the children started going into shock and Gensei forced the scientists to continue gathering data instead of calling for help. Gensei also congratulated Harumi for her work and said he had high expectations for her. Harumi looked at the bloody children, whose heads were covered in blood, and collapsed.
Back in the present, Mikoto lets go of Harumi. The researcher explains that the past experiment had the purpose of studying the overrunning of esper abilities, and the children's bodies were thrown out afterwards. Harumi gets up and resumes the battle. She continues her explanation, revealing that she asked to use Tree Diagram twenty three times but was rejected. She declares that she will do anything to stop such a disaster happening again. At this moment, she screams in pain and collapses while a bizarre embryo-like creature emerges from her head. The creature, the AIM Burst, produces streams of light that damage its surroundings.
Meanwhile, Uiharu Kazari wakes up. She finds that Anti-Skill is unconscious, part of the highway is destroyed and the AIM Burst is floating underneath.