The Universe, also known as the Library, is made up of a series of identical hexagon-shaped rooms. Each room has four walls of books, tiny closet-like spaces for sleeping and using the restroom, and hallways that lead to other hexagons. The hallways contain spiral staircases, which lead up and down to other, identical levels. These hallways also each contain a mirror, which the narrator thinks of as a sign of the Library's infinite nature.
When the narrator was young, he quested in search of a book. Now he is old, and preparing to die. When he dies, someone will throw his body over the edge of a railing and it will fall for eternity.
Each wall of books in the Library contains five shelves, each holding thirty-two matching books. Each book has 410 pages, with 40 lines per page and about 80 characters per line. The cover of each book has a title, but the title has nothing to do with the contents of the book.
The narrator tells us two basic rules about the Library: it has existed forever (and therefore must have been designed by a god), and there are exactly 25 different written symbols. This second point, along with a footnote from a later editor, lets us know that we as readers are outsiders to the universe of the Library, since our world has more than 25 characters.
Most of the books in the Library don't seem to make any sense. The Library's inhabitants used to have a lot of theories about why this is – maybe the books are written in another language, they speculated, or maybe in code. Five hundred years ago, though, a man of genius was able to figure out the Library's secret. Because no two books in the Library are identical, he argued, the Library is "total" – in other words, it contains every single possible combination of the 25 written characters. This means that in all those collections of 410 pages, the Library contains everything that can be expressed in writing, in every possible language.
At first, the reaction to this great discovery was one of optimism and celebration. The people of the Library figured that the truth was out there, and all they had to do was go find it. Men set out in search of their "Vindications," books that tell the stories of their own lives and even tell their futures. Failing to find their own Vindications, these men ended up killing each other off or going insane. Others hoped to find a book that would explain the origin of the Library and of the human race, and established an official group of "inquisitors" to do the job. After several centuries of fruitless searching, though, no one expects to find anything anymore.
The period of optimism was followed by one of despair of ever finding anything meaningful in the Library. Some people suggested they'd have better luck rolling dice and making their own sacred texts. Others, the Purifiers, thought the best course of action would be to destroy all of the meaningless books in the hopes of finding holy ones. But because the Library is so vast, they were unable to make a serious dent in the number of books.
The narrator writes of one lingering suspicion from that period, a godlike figure known as the Book-Man. The idea was that somewhere in the Library existed one book that could explain all of the other books – a "total book" – and that some librarian must have read it. That librarian would acquire the powers of a god. The narrator is sure that the total book must exist, and he hopes that some man has had the chance to read it.
Despite the apparent disorder of the Library, the narrator rejects the idea that any of the books within it are meaningless. There is no combination of letters, he argues, that does not hold a secret significance in some language or code explained somewhere within the Library. The very words he is writing right now, which must already exist within the Library, may have secret meanings that we're unaware of. Are we sure we understand his language?
The human race is now on the verge of extinction, the narrator tells us, though he is sure the Library will endure for eternity. In closing, he explains how he is able to think of the Library as infinite, though it contains a finite number of books. The solution is that it must be periodic, he explains. In other words, it repeats itself: if you start walking in a straight line in any direction, you will eventually come across the same books. The repetition of the disorder of the Library creates an order – the Order, the narrator asserts, of the universe. This belief in an orderly and elegant universe gives him hope.
In a final footnote, the narrator explores an idea of Letizia Alvarez de Toledo, who turns the vast expansion of the Library inside out. The entire Library, in her view, is contained within a single volume with an infinite number of infinitely thin pages. Each page can always be split into two pages. The impossible middle page of this volume would have no back side.