The story tells about a character who mistakenly achieves immortality and then, weary of a long life, struggles to lose it and writes an account of his experiences.

Herein the following five chapters are purported to have been found in the last of six volumes in small quarto (1715–20) of Alexander Pope's Iliad, given to the Princess of Lucinge by a seller of rare books named Joseph Cartaphilus.

Rufus, horrified and repulsed by the city, describes it as "a chaos of heterogeneous words, the body of a tiger or a bull in which teeth, organs and heads monstrously pullulate in mutual conjunction and hatred".

He eventually escapes the city and finds the Troglodyte who followed him there waiting outside; he names him Argos (after the dog of Odysseus), and decides to teach him language.

Rufus wanders the world, fighting at Stamford Bridge, transcribing the voyages of Sindbad the Sailor, and buying the aforementioned edition of Pope's Iliad in 1714.

The story ends with a brief postscript which discusses the fictional book A Coat of Many Colours by Dr. Nahum Cordovero, which argues that the tale of Rufus/Cartaphilus is apocryphal, on the basis of its interpolations of texts by Pliny, Thomas De Quincey, René Descartes and George Bernard Shaw.

The troglodyte who makes patterns in the sand and the hero (Rufus) who finds himself questing after and achieving immortality should be seen as synonymous, all-encompassing representations of the choosing individual within the infinite flux of the universe's permutations.

"The Immortal" has been described as a fictional exploration of Nietzsche's theory of eternal recurrence, in which infinite time has wiped out the identity of individuals.

[2] The Immortal displays Borges' literary irony, fusing Swiftian satire, George Bernard Shaw's creative evolution in Back to Methusela, and the dream visions of Thomas De Quincey in a single work.