Chapter 1 of Toaru Kagaku no Railgun Gaiden: Astral Buddy was released on April 26, 2017.
After several incidents of pranks involving a mysterious culprit, Shirai Kuroko works with a member of Shokuhou Misaki's clique, Hokaze Junko, to determine whether a ghost is the main culprit as the rumors suggest.
Shirai Kuroko responds to a robbing incident and captures the culprit. However, a fellow Judgment member scolds her for moving outside of Tokiwadai Middle School, her division's jurisdiction, and summons Kuroko back to headquarters to deal with a case involving members of Shokuhou Misaki's clique in Tokiwadai.
On the Judgment 177 Branch Office, Kuroko meets Hokaze Junko, a student of Tokiwadai who serves as the #2 in Shokuhou's clique and Rank 5 Level 5's representative in the case. Junko tells them of rumors about a series of incidents involving a "white shadow"—a ghost—but Kuroko treats the rumor with skepticism. Furthermore, most of the victims are Junko's friends, and each incident is left with a card saying "Stay away from my onee-sama!". As a result of this, Kuroko concludes that Hokaze is the "onee-sama" in question, and someone is trying to pull pranks on someone who gets too close to her.
The office suddenly receives a call of a related incident, and Kuroko rushes to the Tokiwadai campus, where a girl was slightly injured after seeing "something white" while using the stairs. Because the incident now involves casualties (albeit minor), Kuroko decides to be more serious on the case. However, in order to catch the culprit, Kuroko plans to stay close to Junko, hoping that she would be targeted next. As Kuroko and Junko begin the plan to expose the prankster, it attacks, throwing a water balloon filled with black ink on Kuroko's face. Although startled at first, Kuroko vows to catch the culprit once and for all. However, without any clues to help them, Junko decided to use her ability to track down the scent from the card left during the latest attack on Kuroko.
As night falls, Kuroko and Junko continue their search inside the campus. The scent trail brings them to a doll filled with irritating perfume. They later continue their search until they see a girl accusing Kuroko of "stealing her onee-sama" and "her indiscriminate affection on other girls" on the school rooftop. Kuroko takes offense to her accusation, but Junko manages to stop the incident from escalating by telling the girl that she would like to be friends with her. The girl who adores Junko is devastated by the response, while Kuroko tries to explain to Junko that the girl is in love with her, much to her confusion. Junko's flustered reaction to the concept causes the girl to lose her footing, and Kuroko is unable to immediately respond due to the perfume interfering with her focus. Instead, Junko leaps off to rescue the girl, using her ability Rampage Dress to boost her physical abilities.
Junko hurts her arm after trying to make a landing, but manages to save the girl with some help from Kuroko. Junko herself is convinced that the incident was caused by the doll and the girl's actions mistaken for paranormal activity. However, as Junko picks up the doll from earlier, a wispy image of a girl appears and greets her.
- Shirai Kuroko resolves a case involving pranks against members of Shokuhou Misaki's clique.
In order of appearance:
- Komaki (unnamed)
- Junko's stalker - a student from Tokiwadai who makes pranks against girls who become too close to Junko, her romantic interest. Due to her admission in Tokiwadai, it can be assumed that she is an esper of at least Level 3.
- Ghost girl
- Rampage Dress (Level 4) - Hokaze Junko
- Kuroko is revealed to be afraid of ghosts.
- The concept of seven mysteries is based on seven rumors of paranormal incidents that involve a particular location. (See also Urban Legends)
- Do ghosts actually exist in Academy City? And can they be explained using Science-side concepts?