"The Disappearance of Haruhi Suzumiya" is a feature-length anime film that serves as a sequel to the 2006-2009 television series "The Melancholy of Haruhi Suzumiya." The story is set in December, shortly after the events of the series, and revolves around Kyon, a high school student and the de facto narrator of the story. Kyon is a member of the SOS Brigade, a club founded by the eccentric and unpredictable Haruhi Suzumiya, who possesses the unconscious ability to alter reality. The brigade includes Yuki Nagato, a quiet and emotionless alien interface; Mikuru Asahina, a time traveler from the future; and Itsuki Koizumi, an esper working for an organization that monitors Haruhi's powers.

The narrative begins with Kyon waking up to find the world drastically changed. Haruhi Suzumiya, the central figure of the SOS Brigade, is nowhere to be found, and her existence has been erased from the memories of everyone around him. Yuki Nagato, previously an emotionless alien, has become a shy and human-like girl, while Mikuru Asahina and Itsuki Koizumi no longer recognize him or recall their involvement with the SOS Brigade. Kyon discovers that the world has shifted to one where Haruhi never gained her reality-altering powers, and the supernatural elements of his life have vanished.

As Kyon investigates, he learns that Yuki Nagato, in her original form, had altered the world to create this new reality. The change was triggered by her desire to escape the endless loop of data collection imposed by her alien superiors, as well as her growing emotional attachment to Kyon. To restore the original world, Kyon must make a choice: return to the chaotic but familiar reality where Haruhi's powers exist, or remain in the peaceful but mundane world where she is an ordinary girl. The film explores themes of identity, choice, and the consequences of altering reality, culminating in Kyon's decision to confront Yuki and restore the original timeline.

The film maintains a focus on Kyon's internal struggle and his relationships with the other characters, particularly Yuki Nagato and Haruhi Suzumiya. It delves into the emotional and psychological impact of the altered reality, providing a deeper exploration of the characters' motivations and personalities. The narrative is driven by a combination of mystery, science fiction, and character-driven drama, offering a more introspective and mature tone compared to the series. The resolution ties together the central themes of the story, emphasizing the importance of human connection and the acceptance of imperfection in life.