A fictionalized version of Borges himself meets an elderly Scottish Bible seller, who hands him a cloth-bound book that he bought in India called The Book of Sand, "because neither the book nor the sand has any beginning or end". Upon opening the book, Borges is shocked to discover that the book, which is written in an unknown language and occasionally punctuated by illustrations, is in fact infinite: if one turns the pages, more pages seem to grow out of the book. He trades his retirement pension and a Wycliffe Bible to the bookseller in exchange for the Book of Sand and places it on a bookshelf behind his copy of Arabian Nights.

Over the summer, Borges obsesses over the book, poring over it, cataloging its illustrations, and refusing to go outside for fear of its theft. In the end, upon realizing that the book is a monster, he briefly considers burning it before fearing the possibility of the smoke of an infinite book suffocating the world. Instead, he goes to the National Library to hide it in one of the basement bookshelves, reasoning that "the best place to hide a leaf is in a forest".