The Disappearance of Haruhi Suzumiya (涼宮ハルヒの消失) (Suzumiya Haruhi no Shōshitsu) is a Haruhi Suzumiya Light Novel.
What if you woke up one morning, and everything changed?
It's one week before Christmas Eve, and Haruhi and the S.O.S. Brigade (a club for her high school's strangest and most extraordinary students) are gearing up for holiday festivities. But just before the fun kicks off, Kyon, the only "normal" member, wakes up in a weird alternate dimension, one where Haruhi attends another school entirely, Nagato the time traveling robot is just an ordinary human, and Mikuru (the cute girl of Kyon's dreams) doesn't even recognize him—in other words, S.O.S. Brigade never existed.
The only clue Kyon can find is a bookmark left by the robot version of Nagato, which leads him on a quest back in time, where he interacts with the storyline from "Bamboo Leaf Rhapsody", a short story from the previous Haruhi book, The Boredom of Haruhi Suzumiya.
This fun and quirky holiday tale is reminiscent of A Christmas Carol and It's a Wonderful Life.
On December 17, Haruhi organizes a Christmas Eve event in which Mikuru will dress up in a Santa suit and Haruhi will serve hotpot; Taniguchi announces that he has a date from Kouyouen, an all-girls school for the rich and famous. However, Kyon remains unprepared for what happens the next day...
On December 18, Kyon sees Taniguchi walking up the hill, wearing a face mask. Not only does Taniguchi claim to been having a cold the previous day before, he also denies having a date at all. Confused, Kyon continues to school, where he finds that several of the other students have colds as well where there had been none before.
In class, Kyon notices that Haruhi does not come at all, and starts to grow increasingly worried. Without warning, Ryoko Asakura appears, saying that she had been sick during the morning but felt well enough to come. Kyon questions why she is there, but everyone looks at him, puzzled, claiming she had been there the whole year. When Kyon asks about Haruhi, nobody seems to understand, and when he asks for the class list, Haruhi's name is not on the list.
In shock, during lunchtime, Kyon runs to the Yuki's class to find her, but she is gone; he runs to Itsuki's class to find that the entire classroom has disappeared; he finds Mikuru and Tsuruya in the hallway to find that they do not remember him and Mikuru knows nothing about being a time traveler.
In a last attempt, Kyon runs to the Literature Club room, and finds Yuki there; however, she is wearing the glasses she stopped wearing in The Melancholy of Haruhi Suzumiya, she denies being an alien, and she is showing clear emotions different from what Kyon remembers, instead being simply a shy and introverted human girl. All of the items kept by the SOS Brigade are missing, and the computer is not the new model Haruhi stole but instead an old, decrepit computer with no signs of the files that were stored in there. As Kyon leaves the room, Yuki asks him to join the Literature Club.
When Kyon gets home, he tries to get Shamisen to talk to him the way he did in The Sigh of Haruhi Suzumiya, but to no avail. Disappointed, Kyon waits until the next day, December 19, to inquire further. The world is still altered in that Haruhi and Itsuki have disappeared, and Mikuru and Yuki have no memories of the SOS Brigade.
Kyon returns to the Literature Club room and finds the book that Yuki had once lent him with a bookmark telling him to meet her. (This bookmark had led to Yuki revealing her identity to him as an alien in The Melancholy of Haruhi Suzumiya.) Kyon opens the book and sees another bookmark in it, and finds a message from the Yuki he once knew: "Program Run Condition: Collect the keys. Deadline: Two Days Later."
Unsure about what the message means, Kyon inquires into Yuki's home and who she lives with. The human Yuki, like the former Yuki, lives alone. She invites Kyon to her apartment. When they arrive, Kyon realizes that Yuki has one memory of him: an incident from The Melancholy of Haruhi Suzumiya in which he helped her make a library card. However, instead of a brigade member trying to get her out of the library to meet Haruhi, Yuki remembers him as a random passerby who helped her.
Ryoko Asakura comes to visit Yuki, and when Kyon tries to back out, Yuki pulls on his sleeve lightly. Kyon is moved by Yuki's sudden display of expression, and stays behind even in the presence of Ryoko. Kyon discovers that the Ryoko in this world is only a friendly neighbor trying to help Yuki rather than backup.
The next day (December 20), Kyon mentions Haruhi in his PE class, and Taniguchi recognizes the name. Because he went to Haruhi's middle school, he knows who she is, and tells Kyon that she is currently at Kouyouen School, which had been turned from an all-girl's school to a school for the elite. Stunned, Kyon skips school to find Haruhi at Kouyouen. As it turns out, he not only finds Haruhi but also several other people from Itsuki's class, including Itsuki himself; in fact, Itsuki even walks home with Haruhi in the new world.
Kyon finds Haruhi and tries to speak to her, but she pushes him off because she does not remember him. Kyon then asks her about the incidents at the Tanabata of three years ago, and when she realizes that he knows of all of that, he tells her that his name is "John Smith", the alias that he used during the "Bamboo Leaf Rhapsody" incident.
A stunned Haruhi then listens to everything Kyon says about his alternate universe, Yuki being an alien, Itsuki being an esper, Mikuru being a time traveler, and Haruhi having strange powers. Intrigued, Haruhi decides to gather the former SOS Brigade members and meet them.
Kyon does not fail to notice that Itsuki is catering to Haruhi's every whim, and inquires for the reason. Itsuki admits that he does "fancy Suzumiya-san", and thinks she is a captivating person. While Haruhi rushes over to North High with Itsuki and Kyon, Kyon tries to understand what Itsuki would see in her, fails, and shrugs it off quickly.
Haruhi borrows Kyon's physical education clothes and poses as a student at North High. Pretending to be in the Student Council, she takes Mikuru from the Calligraphy Club and brings her to the Literature Club room. Once they enter the room, the computer activates and displays a message from the original Yuki. The message explains that there is a program in the computer: if Kyon chooses to activate it, he will find a way out of the new world; if he chooses to deactivate it, he will remain. Kyon immediately chooses to activate it.
Instead of returning to his original universe, however, Kyon finds himself in the Tanabata of three years ago, and realizes he must enlist the help of the older Mikuru and the Yuki of the past in order to return. He finds the older Mikuru, who directs him to Yuki's house, and Yuki explains that a certain person altered the world on December 17, changing everyone's memories within one year. As Yuki explains this, she injects nanomachines into Mikuru and Kyon to become invisible to the culprit and gives them a weapon to return the culprit to normal. When Kyon asks the name of the culprit, Yuki reveals it to be herself; that is, the herself of Kyon's time. Yuki had altered the world because of "errors", what Kyon later deems to be emotions that she couldn't understand. Yuki had only preserved Kyon's memories in order to let him make the choice.
Kyon and Mikuru travel to December 18, and reach the human Yuki. Kyon apologizes to the human Yuki, who is confused, and ponders his choice to go back to his old world, thinking that he could have stayed at the Literature Club for good and joined the SOS Brigade at the same time, allowing Yuki to keep her wish. However, he concedes that his old world, with supernatural beings, was a much more interesting place.
As Kyon is about to change Yuki back, however, Ryoko intervenes and stabs Kyon with a knife, thinking that he means to kill Yuki. Kyon's vision starts to blur and he sees the younger Mikuru and the alien Yuki intervening, along with a mysterious person who tells him not to worry. Kyon then falls unconscious.
Kyon wakes up in a hospital run by Itsuki's 'Agency', and finds that the world has been turned back to normal. According to the SOS Brigade members of his universe, after discussing plans for Christmas, Kyon fell down the stairs and went into a coma for three days, and apparently Haruhi has stayed by his side for those three days.
Kyon wonders who the mysterious person who had told him not to worry and had rectified the world was, and finally realizes that it was himself, a near-future version of himself, who had gone with Mikuru and Yuki to save the world as he knew it.
Yuki visits Kyon that night and tells him that her punishment is being assessed by the Data Overmind for altering the world, because it is afraid she will make further errors. Kyon, outraged, grabs Yuki's hand and makes a threat to the Data Overmind: if they do anything to Yuki, he will do whatever it takes to get her back, which would include telling Haruhi that he is John Smith and making her change the world so that Yuki will exist but the Data Overmind will not, because Yuki is his friend. Yuki thanks him for doing so.
On Christmas Eve, Kyon considers his duty to go back in time and save himself, but decides not to do it then, and decides to go have some of Haruhi's hotpot.
A spin-off series of the story was made called The Disappearance of Nagato Yuki chan which is about a world in which Kyon hadn't kept his memories of the original world and did not activate the escape program.