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; Kyon invites Taniguchi but he declines, saying he has a date from Kouyouen, an all-girls school for the rich and famous.
On December 18, Kyon sees Taniguchi walking up the hill, wearing a face mask. Taniguchi claims to been having a cold since the previous day, and denies having a date at all. Confused, Kyon continues to school, where he finds that several other students have colds as well, where none had before.
In class, Taniguchi's illness eventually forces him to go home, and Haruhi does not show up at all. Instead, Ryoko Asakura arrives during lunch break, saying she had been sick during the morning but felt well enough to come. When Kyon questions her presence, everyone says she has been in Haruhi's seat since the last seat reassignment. When Kyon asks about Haruhi, nobody remembers her, and Asakura shows him Haruhi's name is not on the class list. Kyon runs to Yuki and Itsuki's classes, but Yuki is not there and Itsuki's entire classroom has disappeared. He finds Mikuru and Tsuruya in the hallway, but they do not remember him and Mikuru knows nothing about being a time traveler.
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, denies being an alien, and behaves differently 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 an old, decrepit model with none of the files that were stored in there. As Kyon leaves the room, Yuki gives him an application 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. The next day, December 19, Kyon returns to the Literature Club room and finds the book that Yuki lent him in The Melancholy of Haruhi Suzumiya. Kyon opens the book and sees another bookmark in it, with a message in Yuki's handwriting: "Program Run Condition: Collect the keys. Deadline: Two Days Later."
Unsure 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, Yuki tells him that they met once before, when he helped her get a library card. Kyon notes this is similar to an incident that occurred in The Melancholy of Haruhi Suzumiya. Ryoko Asakura comes to visit Yuki. Kyon tries to back out, but Yuki pulls on his sleeve lightly. Kyon is moved by Yuki's display of expression, and stays behind even in the presence of Ryoko. After dinner, Ryoko privately warns Kyon to be sensitive to Yuki, assuming he is romantically interested in her.
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 went to Kouyouen School, which he says is now co-ed. Kyon leaves school and waits outside Kouyouen for classes to let out. Among the students, he spots Haruhi walking with Itsuki. Kyon approaches her, but she pushes him down because she does not remember him. Kyon then tells he is "John Smith" from the "Bamboo Leaf Rhapsody" incident.
Her interest caught, Kyon tells her everything about his alternate universe, Yuki being an alien, Itsuki being an esper, Mikuru being a time traveler, and Haruhi having strange powers. Excited, Haruhi demands they gather the former SOS Brigade members so she can meet them.
Kyon asks Itsuki about his relationship with Haruhi. Itsuki says that he is very attracted to her, but she is already growing bored of him because he is so ordinary. So they can get into North High, Haruhi and Itsuki don Kyon's physical education clothes. 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 the five members of the SOS Brigade are the keys and there is a program in the computer: if Kyon chooses to activate it, he will find a way out of the new world; otherwise, he will remain. Kyon realizes that since they have now reformed the SOS Brigade, the only important difference between the old world and the new one is that they are all normal humans. He nonetheless chooses to activate it, conceding that his old world, with supernatural beings, was a much more interesting place.
The program takes Kyon to the Tanabata of three years ago, and he 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. Yuki explains that because of "errors", she stole Haruhi's power and altered the world on December 17, changing everyone's memories within one year. Yuki injects nanomachines into Mikuru and Kyon to bypass her detection and gives them a weapon to return the world to normal.
Kyon and Mikuru travel to December 17, and reach the human Yuki. Ryoko intervenes and stabs Kyon with a knife, claiming Yuki remade her with the sole purpose of protecting Yuki from any threats. 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 Itsuki, after discussing plans for Christmas, Kyon fell down the stairs and went into a coma for three days, and Haruhi has stayed by his side for those three days, taking her sleep in a sleeping bag by his bed. He apologizes to Haruhi for worrying her.
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, makes a threat to the Data Overmind: if they do anything to Yuki, he will tell Haruhi that he is John Smith, and Haruhi will change the world so that Yuki will exist but the Data Overmind will not, because Yuki is her friend. Yuki thanks him for doing so.
Kyon realizes the person who told him not to worry must have been a near-future version of himself, who went with Mikuru and Yuki to save the world. On Christmas Eve, Kyon considers his duty to go back in time and save himself, but decides not to do it then, and goes to have some of Haruhi's hotpot.
Kyon considering drawing on Haruhi's face while she sleeps is a callback to "Remote Island Syndrome", in which he threatens to draw on her face if he ever catches her sleeping.
A spin-off series was made called The Disappearance of Nagato Yuki chan which is loosely based on the altered universe seen in The Disappearance of Haruhi Suzumiya.