Warning: Wikipedia contains spoilers
Pale Fire is to all appearances the publication of a 999-line poem in four cantos ("Pale Fire") by the famous American poet John Shade. The Foreword, extensive Commentary, and Index are by Shade's self-appointed biographer, Charles Kinbote, who is Shade's neighbor in the small college town of New Wye.
Shade was murdered and left the poem unfinished. Kinbote has taken it upon himself to oversee its publication, telling readers that it lacks only one line.
In the Commentary and Index, Kinbote concentrates surprisingly little on explicating the poem. Instead he tells his own story and the story of Charles Xavier, the deposed king of the "distant northern land" of Zembla. The reader soon realizes that Kinbote is Charles Xavier, living incognito--or that he is insane and his identification with Charles and perhaps all of Zembla are his delusions.
Interpretations of the book are heavily debated. Some readers concentrate on the apparent story (a minority believe that Zembla is as "real" as New Wye), focusing on traditional aspects of fiction such as the relationship among the characters. They may make a case that Kinbote is parasitic on Shade, or that Shade's poem is mediocre and Kinbote, the inventor of Zembla, is a true genius. In 1999, Brian Boyd published a much-discussed study arguing that the ghost of Hazel Shade, the poet's suicided daughter, influenced the commentary as well as the poem itself, and that the ghost of John Shade influenced Kinbote's contributions.
Other readers see a story quite different from the apparent narrative. "Shadeans" maintain that John Shade wrote not only the poem, but the commentary as well, having invented his own death and the character of Kinbote as a literary device. "Kinboteans", a decidedly smaller group, believe that Kinbote invented the existence of John Shade. Some see the book as oscillating undecidably between these alternatives, like the drawing that may be two profiles or a goblet.
Still other readers de-emphasize any sort of "real story" and may doubt the existence of such a thing. In the interplay of allusions and thematic links, they find a picture of English literature, criticism, or some other topic.
The only consensus is that the book is unique.