Therestina reminisces shortly on her experience as First Sample before she receives a report from her subordinates. Outside, she watches as Kiyama Harumi continues her pursuit on the vehicles containing her students. A road block is set up behind her and she watches as MAR units equipped with the HsPS-15 mechanized armor emerge from the vehicle in front of her.
Suddenly, the few MAR vehicles in front of Kiyama's car were blown away from the force of an impact. It was Mikoto, using her electricity to unleash a force strong enough to immobilize MAR's armored cars. Kuroko also appears. Kiyama wanted to question why they were there when Mii arrives on a motorcycle, with Saten and Uiharu as passengers. The two quickly hops in to Kiyama's car and volunteers to help navigate, telling Harumi that the MAR vehicles are decoys and the children are elsewhere.
Kiyama speeds off to pursue Therestina as MAR units emerge from the ruined cars. A few helicopters from MAR become reinforcements. Kuroko feels she and Mikoto are outnumbered in terms of firepower. Kongou Mitsuko suddenly appears, using her ability Aearo Hand, to throw off one of the wrecked cars of MARs into the helicopters. Kuroko teleports Mikoto onto the back of Mii's motorcycle and tells her that she and Mitsuko will handle the HsPS-15 units.
With an even larger number of metal pins than she usually uses, Kuroko proceeds to disable the artillery weapons of the mechanized units while Mitsuko throws them off using her ability. Kuroko, usually at odds with Mitsuko's attitude, remarks that their team-up was unexpected.
Meanwhile, Mii manages to catch up to Kiyama's car. Inside, Uiharu breaks the news that the target is disguised as a civilian vehicle. They soon receive a surprise attack from Therestina from below; it lifted Mii and Mikoto off the ground. Mii manages to throw off Mikoto and the Electromaster uses her electricity to reach Kiyama's car. Behind them, Therestina proceeds to finish them off, equipped with an even larger robotic vehicle to pursue them.
In the middle of the tension, Mikoto apologizes to Kiyama for making a mistake. Kiyama responds that she would do the same thing if their roles are reversed.
After the apology, Mikoto prepares to go full-force against TelesTherestinatina. She fires a lightning bolt straight to Therestina, but was repelled. Therestina gloats on knowing about her ability as Mikoto fires a Railgun, which also fails to do damage. Therestina reveals that she knew about her ability, especially its weaknesses. Therestina fires a grappling arm at Mikoto, which she manages to dodge with the help of Kiyama.
As Kiyama heads off to a junction, Therestina calls one of his subordinates, which reported being under fire. In fact, it was Anti-Skill units holding off MAR's units from proceeding.
Therestina decides to do things her own way, attacking Kiyama's car as she proceeds into the destination. Harumi manages to evade Therestina's attacks. Therestina fires her grappling arm to Mikoto once more; she manages to reach the Electromaster, but her grappling arm ends up destroyed by Mikoto's electricity. Therestina shrugs off Mikoto's attack and charges to her; Mikoto calls Kuroko, who teleports a large part of Therestina's grappling arm to Mikoto's range. She uses this to unleash a more powerful version of her Railgun; it tore through the massive robot and left it incapacitated.
Uiharu and Saten emerge from Kiyama's car and asks in they are alright. Mikoto looks at Kiyama, who interrupts her remark about saving, telling her to say it after she saves the children. She agrees.
In District 23, as Uiharu and Kiyama proceeds to hack through the laboratory's security protocols and later proceeds to the lowermost level of the facility, where they assume the children are located. There, they discover the children and Haruue Erii, who are unharmed.
The reunion is interrupted by the sound of Capacity Down, which affected everyone except Kiyama. Therestina also appears, bloodied from Mikoto's attack earlier. She attacks Mikoto and Kuroko using the HsPS-15 unit and retaliates from Kiyama's attempt to rush to her. Therestina approaches Uiharu while Saten watches from a distance, holding the aluminum bat she prepared to use before they reached District 23. Uiharu indirectly tells Saten to hurry to the central control room, where she suspects the Capacity Down is controlled.
Therestina explains her plans of using Haruue Erii as the first Level 6 to be created by the SYSTEM project. She tells that Erii's potential will be the central aspect of the experiment, and she has no qualms in continuing despite destroying Academy City in its wake. Also, she remarks that Academy City and all of its inhabitants are mere guinea pigs for the project, which enraged Saten, who watches over the events in the control room while she tries to disable Capacity Down.
With her aluminum bat, Saten decides to destroy the systems inside the control room, managing to disable Capacity Down in the process. Mikoto manages to use her powers again and uses it to throw Therestina off, causing her to let go of First Sample. Kiyama manages to pick up First Sample and watches as Therestina unveils her newest weapon, a copy of Mikoto's Railgun which she proclaims to be stronger than the original. Mikoto calmly prepares a coin for her Railgun as Therestina prepares to fire hers; the Electromaster remarks on Academy City being a perfect place for all of them. The two Railguns face off, and a large explosion rocks the facility.
Later on, Saten returns to see Uiharu assisting Erii. She is then congratulated by Kuroko. Mikoto watches as Kiyama prepares the final stages to complete her mission to revive her students. Erii assures Kiyama in proceeding, telling her that her friend Banri believes in her. With a single keystroke, she ends the SYSTEM project.
For the first time, Kiyama Harumi reunites with her students as Erii sees her friend's face again after a long time. Kiyama then looks at Mikoto and finally says thanks. Mikoto's face blushes and she smiles back.
After the events... Mitsuko tells her experiences in battle to Kinuho and Maaya. Meanwhile, the ten Child Errors and Erii wait for Mikoto and the others. In fact, they overslept last night, and were hurrying to reach Erii in time. Kuroko also receives news from Mii that Therestina is regained consciousness, though she is not willing to give information.
Just as Kuroko finishes her conversation with Mii, one of the airships giving out information flashes an announcement video: it was the Child Errors handing out a "Happy Birthday" message to Kiyama Harumi, who was also recovering in the hospital. Teary-eyed from the message, she smiles as she looks out into the sky, along with all of the minor characters from the series and characters from Toaru Majutsu no Index who appear in cameos.
Rating: 8/10