The Dissociation of Haruhi Suzumiya (涼宮ハルヒの分裂) (Suzumiya Haruhi no Bunretsu) is a Haruhi Suzumiya Light Novel.
As the second year of high school begins, Haruhi's cynical and sarcastic confidant Kyon, the only "normal" member of the SOS Brigade, finds himself meeting Sasaki, an old friend from his days in junior high. Haruhi and Sasaki's shared friendship with Kyon and, possibly, shared abilities, causes tension between the two and Kyon knows Haruhi's dissatisfaction can have dire consequences! Kyon and Sasaki's chance meeting could affect his future, the future of the SOS Brigade, and the world forever.
This ninth volume of the Haruhi Suzumiya series is the first to use two alternate stories for the same plot with different endings. The story continues in the next novel, The Surprise of Haruhi Suzumiya.
Kyon enters his second year of high school at North High and notices that the ordering of the classes is almost identical to the previous year; Kyon is still grouped with Haruhi in Class 2-5, while Itsuki is Class 2-9 and Yuki is in a separate class. Haruhi forces Mikuru and herself into cheongsams, while the president of the Student Council comes to investigate that the Literature Club is in session and not the SOS Brigade. Haruhi, however, fools him by keeping a layer of wooden foil on top of a sign that says "SOS Brigade" to make it seem like it says "Literature Club". The Computer Study Group president visits the SOS Brigade and tells Kyon that the few times Yuki visits she has been very helpful, fixing every bug and even creating a new OS that everything is compatible with. Haruhi announces her plans to film the sequel to "The Adventures of Mikuru Asahina Episode 00", "The Revenge of Yuki Nagato Episode 00".
Itsuki says that Haruhi has become more unstable due to an incident that had happened earlier. On the way to the train station, Kyon met an old middle school friend of his, named Sasaki. Sasaki mulled over certain things in life, leading Kyon to believe she was much like Itsuki. The rest of the SOS Brigade arrived at the spot, and when Sasaki introduced herself as a "good friend" of Kyon's, Haruhi was taken aback since Kyon had never mentioned Sasaki to her.
Kyon met Sasaki in cram school. They would often talk about trivial things like Kyon's name, and ride on bicycle from middle school to cram school together and Kyon would escort Sasaki to her bus station.
On Saturday evening Kyon meets Sasaki again, this time accompanied by the girl that kidnapped Mikuru in The Intrigues of Haruhi Suzumiya, Kyouko Tachibana. Incensed, Kyon tries to dissuade Sasaki from associating with Kyouko. Sasaki tries to calm him down and introduces Kyon to another friend of hers, Kuyou Suou, a humanoid interface from an entity different from the Data Overmind (the Macrospatial Quantum Cosmic Existence, mentioned in "Snow Mountain Syndrome" and The Intrigues of Haruhi Suzumiya) with deficient communication skills (she can barely convey her own name), seemingly stemming from a lack of individuality. Sasaki also reveals that she has been associating with the future man from The Intrigues of Haruhi Suzumiya, and tells Kyon she knows about their true identities; Kyon only confirmed it.
The SOS Brigade runs into Kyon, Sasaki, and her associates, and Kyon notices tension between Haruhi and Sasaki, Yuki and Kuyou, and Itsuki and Kyouko. Kyon goes home and gets a phone call. The plot splits from here:
The caller is a girl unknown to Kyon, but who sounds extremely familiar. After saying some strange things (such as suggestion Kyon should not have forgotten her) she hangs up. Kyon calls Itsuki and asks about Kyouko and Sasaki's other associates, and Itsuki replies that even though they possess similar values as the SOS Brigade, Kuyou Suou presents a threat as nobody can understand her motives.
Kyon wakes up the next day, a Monday, and meets up with Haruhi, who tries to persuade him to take more action in brigade activities, because she is recruiting new members who may surpass him. Kyon also speaks with Yuki, who confirms that Kuyou's motives are unknown, but her faction was responsible for her illness in Snow Mountain Syndrome. She also explains that the Data Overmind has named the Macrospatial Quantum Cosmic Existence the "Sky Canopy Dominion", because they come from the zenith of the sky.
At the end of class, Haruhi helps Kyon prepare for an upcoming math quiz. Haruhi welcomes new first-year brigade applicants to the room, all of whom Itsuki confirms to have no extraordinary origin. Although Kyon counts eleven members at the beginning, someone slips in, changing the count to twelve. After Haruhi kicks out a potential member because he doesn't know what the SOS Brigade does, Haruhi declares that all of the potential members are in an "entrance examination". Kyon notices there is one girl who is taking a great amount of interest in the Brigade and what it does, and experiences a nagging feeling that he's seen her before. Haruhi dismisses the new members, saying that anyone who wants to join the Brigade attend the next day, as she does not want anyone who bails to join her Brigade. As the first-years leave and Haruhi prepares a written examination, Kyon wonders who will return the next day.
The caller is Sasaki, who invites Kyon to meet her, the time traveler man, Kyouko, and Kuyou at the café the SOS Brigade usually meets at Sunday morning. After Kyon hangs up, he calls Itsuki, who believes that Kyouko does not acknowledge Haruhi as a god and cannot enter closed space. Kyon also calls Yuki and asks if it is safe to attend the meeting, and she confirms it, also revealing to Kyon that the Data Overmind has created a more accurate name for the Macrospatial Quantum Cosmic Existence: the Sky Canopy Dominion.
Kyon attends the meeting at the arranged time, where he meets the time traveler man, who introduces himself using a fake name, "Fujiwara". Kyouko tells Kyon her organization believes that Sasaki is the god and not Haruhi. Because Sasaki is emotionally stable, Kyouko believes that Haruhi's powers should be transplanted to Sasaki in order to avoid danger, even though Sasaki does not wish to be a god. To prove her point, Kyouko takes Kyon into Sasaki's version of sealed reality, which is a white version of the world where Avatars are completely absent. (However, Kyon notices that they are devoid of any change at all.) After Kyon and Kyouko return, they come upon Emiri Kimidori, who serves as the waitress at the café. Kuyou grabs onto her wrist for no apparent reason, and abruptly leaves after letting them go. The meeting breaks up over disagreements, as only Kyouko is actually interested in transferring power to Sasaki.
At school the next day, a Monday, Kyon thinks back to a conversation he once had with Sasaki about whether aliens, time travelers, or espers exist; Sasaki vehemently denies their existence and gives Kyon a logical argument as to why they cannot. When Kyon asks about what Sasaki thinks about attractiveness, Sasaki answers that she believes love is much like a mental illness and is meant more for instinct as to preserve the human race.
At the SOS meeting, Kyon notices that Mikuru is starting to look more like the future Mikuru he has seen in The Melancholy of Haruhi Suzumiya, and believes this signifies that he will be parting with Mikuru soon. Haruhi comes in and tells Kyon that she will be holding examinations for potential first-year applicants to the SOS Brigade. Haruhi noticed that Yuki is absent. The Brigade looks for Yuki in her classroom and the Computer Study Group room, but she is gone. Haruhi calls Yuki, and Yuki tells Haruhi that she has the flu, something Kyon believes is the responsibility of Kuyou Suou and the Sky Canopy Dominion, while Itsuki believes it to be the responsibility of the silicon lifeforms found in "Wandering Shadow". Kyon considers whether to tell Haruhi that he is John Smith while Haruhi organizes the Brigade to set off for Yuki's apartment.