The title is derived from lines in Sir Richard Francis Burton's poem The Kasîdah of Hâjî Abdû El-Yezdî: Reason is Life's sole arbiter, the magic Laby'rinth's single clue:Worlds lie above, beyond its ken; what crosses it can ne'er be true.Originally intended to be the final book in the series, this book continues the chronicles of the adventures of Sir Richard Burton, Cyrano de Bergerac, Alice Liddell, Tom Mix, and Samuel Clemens through a bizarre afterlife in which every human is simultaneously resurrected along a single river valley covering an entire planet.

Through his internal reverie he reveals that his identity was discovered by Monat Grrautut, the director of the Riverworld project, who recalled 'X' to the Dark Tower to be judged.

Eventually, the two riverboats reach Virolando, a wide lake before the headwaters, home to the pacifist Church of the Second Chance, where Hermann Göring has become a priest.

There, King John and Clemens begin an aerial dog-fight, and later a naval combat in which both riverboats are sunk and most of the crews die.

Among the survivors are Burton, Frigate, Alice, Kaz, Joe Miller, Li Po, Ah Qaaq, and Nur ed-Din ("Light of the Faith"), now joined by Tom Turpin, Aphra Behn, a man claiming to be Gilgamesh, and Baron de Marbot.

The protagonists discover a malfunction in the computer which runs the tower, threatening to release the stored wathans (souls), of the people who died on the Riverworld.