This ambitious work melds the "school horror" genre with a classic puzzler mystery. Author Yukito Ayatsuji himself has called it a new standout among his works.
The story begins in May 1998, when narrator Kōichi Sakakibara moves from Tokyo to his grandparents' house and begins attending ninth grade at the same school his mother, Ritsuko, had gone to. Kōichi is in treatment for a lung condition, and has been sent to live with his grandparents while his college professor father Yōsuke is away in India for an extended stay in connection with his work. Ritsuko died 15 years ago at the age of 26, the same year she gave birth to Kōichi.
Class 9-3, to which Kōichi is assigned, has long been regarded as a "haunted classroom" by the locals. In May 1972, a boy named Misaki Yomiyama died in a house fire that killed his entire family of four. Misaki was an honor student and had a cheerful nature that made him a class favorite; mourning his untimely death, all of his classmates and his teacher acted as if Misaki were still with them for the rest of the school year. Impossibly, Misaki appeared in the group photo of the class taken at the end of the school year.
Even greater misfortune came to Class 9-3 the following year, when the number of classmates and family members who died totaled 16. Calamity has not struck every year since then, but in the years when it has, there have been many fatalities. A common characteristic of the ill-fated years is that someone joins the class partway through the year, but by the time the school year comes to an end, neither the teachers nor even the students remember that person. This mysterious identity is said to be someone who died in a previous "year of misfortune."
The students in the Class of 1998 attempt to head off calamity by preemptively choosing someone to play the role of the extra classmate, and agreeing to act as if that person isn't there. The idea is that this will cancel out any other extra person who might appear, thus offsetting the curse. Mei Misaki, a girl who wears a patch over her blue glass eye, is chosen for the role. Kōichi is unaware of the curse on his class when he gets to know Mei at the hospital and engages her in conversation without thinking anything of it. But perhaps because this violates the agreement to act as if Mei doesn't exist, a Class 9-3 student's mother dies in a traffic accident, and then, in making too much haste after hearing of her mother's death, the girl falls down some stairs and dies when an umbrella tip impales her throat. Two more people die in June, and three more in July. It is clear that another year of misfortune has begun.
At the end of July, a cassette tape is found in an old classroom at the school, hidden there by a 9-3 student from the Class of 1983. The student says on the tape that the misfortune stopped when he accidentally killed the extra person. Does this mean that they can put an end to their own year of misfortune by figuring out who the extra person is and killing him or her?
In August, the class goes on an excursion to the school's retreat center at the foot of Mt. Yomiyama. The objective is to visit the Yomiyama Shrine and have the misfortune afflicting the class exorcized. At night, with mobile phones suddenly unable to get a signal, tragedy occurs. Of the couple serving as caretakers of the facility, the wife goes berserk and attacks first her husband and then the students, one after another, with a knife, killing six in all; she then bites through her tongue to kill herself as well.
During the rampage, Mei, whose glass eye sometimes sees unusual things, discovers who the extra person is, and Kōichi kills her with a pickaxe. The extra person is the teacher's assistant, who it turns out is Ritsuko's younger sister by 11 years. She had died in the year of misfortune that struck two years before, when she was teacher of the class.
This work dramatically expanded Ayatsuji's fan base, especially among younger readers. It became a bestseller and was adapted into two different anime versions (television and original video) as well as a live-action film. A sequel appeared in 2013 titled Another: Episōdo S (Another: Episode S), and Ayatsuji has indicated he has plans for two additional volumes.