There are six main plot lines that the player will have the chance to experience, one for each of the heroines in the story. Throughout gameplay, the player is given multiple options to choose from, and text progression pauses at these points until a choice is made. To view all plot lines in their entirety, the player will have to replay the game multiple times and choose different choices to further the plot to an alternate direction. In the adult versions of the game, there are scenes with sexual CGs depicting Kōhei and a given heroine having sex. Later, ports of the game were released without the erotic content.
Plot
Setting
The main part One's story takes place at a Japanese high school, though little is shown except for Kōhei's classroom, the cafeteria, and a little of the surrounding area. The main school building is three-stories, and the roof is accessible, albeit with a sign on the roof's access door saying "No Entry", but the door is never locked regardless. Adjacent to the main building is another which houses club rooms. The school borders a forest on one side with a chain link fence. Beyond the forest is a park with a fountain and a flight of stairs on a hill. Other than the school, the shopping district in town and the usual path Kōhei takes to school are shown, but otherwise the only other place depicted is Kōhei's two-story house.
Main characters
Kōhei Orihara is a young man in high school living a normal life. He tends to tease the girls he knows, especially his childhood friend Mizuka Nagamori, but deep down he is generally nice to girls. Mizuka usually takes good care of him, and worries about Kōhei, wondering if he could suddenly get a girlfriend. She is constantly sighing, being made to go along with Kōhei's meaningless jokes. Kōhei meets a transfer student named Rumi Nanase early in the story whose goal is to become a "true maiden" and to that end, she behaves maidenly toward those around her, but she shows her true character just to Kōhei, who leaves a terrible first impression. Kōhei gets to know a quiet classmate named Akane Satomura who does not readily open her heart up to those around her, and refuses any help Kōhei may offer.
Kōhei also meets a blind upperclassman named Misaki Kawana who has a sociable personality where she can be unreserved with anyone. A mute underclassman girl named Mio Kouzuki meets Kōhei one day and uses a sketchbook to carry on written conversations. She belongs to the drama club, and despite not being able to speak, can still show emotion through various abundant expressions. The last girl Kōhei gets to know is a childish junior high school student named Mayu Shiina who refuses to go to school. Her one and only friend she could trust in life, a ferret named Myū, dies shortly before she meets Kōhei and the other main characters.
Story
The story revolves around Kōhei Orihara, the main protagonist. His father died when he was very young, followed by his younger sister Misao and finally his mother. Deeply depressed, he goes to live with his aunt Yukiko, his mother's younger sister, about ten years before One's story begins on November 30, 1998. After he meets Mizuka Nagamori as a child, Kōhei begins to seclude himself in the Eternal World, a world within his mind. The game's main recurring motif is a focus on the Eternal World, a mystical alternate space which is never clearly explained, and the details of which are unknown. Shun Hikami's remarks in the original game are possible clues about the Eternal World.
The Eternal World is a place similar to the afterlife where a person's "other self" awaits. Anyone can access it, though it is only accessible once one has lost his or her grounding in the real world. It may be necessary to form a pledge with someone in the real world as a guide to the Eternal World, but one's memories surrounding the pledge become vague. A grace period can be granted between when the pledge is made and when someone goes to the Eternal World. Once the process has started, nothing can prevent someone from going to the Eternal World, and returning to the real world is difficult. Someone about to leave to the Eternal World starts to be forgotten approximately one week before going, and the amount of time before someone forgets differs based on how much someone thinks about the person leaving. However, he or she is remembered the moment that person returns. If a strong emotional bond is established in the real world before leaving, a person in the Eternal World can be returned after approximately one year.
Kōhei is living a normal life in high school with Mizuka, and gets to know five other girls throughout the story who he helps with their various personal problems. While Kōhei likes to joke and tease these girls, deep down he is nice to girls and genuinely wants to help them. In the story, Kōhei is able to form intimate relationships with the six heroines: Mizuka Nagamori, Rumi Nanase, Misaki Kawana, Mio Kouzuki, Mayu Shiina, and Akane Satomura. While a given relationship will start out well, before long those around him start to forget him as he begins to retreat further into the Eternal World. If Kōhei forms a strong bond with one of the girls, she alone remembers him even after he leaves to the Eternal World. This ensures his eventual return one year later when he and the girl reunite.
