This novel follows three intellectual friends who work at a small publishing house. As a joke, they start inventing a conspiracy theory about a secret society that has been manipulating world events for centuries. However, as they delve deeper into their own fabrication, they begin to lose sight of what's real and what's not. Their lives take a dangerous turn when actual secret societies believe they hold the key to a universal secret and will stop at nothing to obtain it.

The book is a contemplative exploration of the world through the eyes of the titular character, Mr. Palomar, who seeks to understand the complexities of existence through meticulous observation and reflection on seemingly mundane subjects. From watching waves on a beach to contemplating a cheese shop, his attempts to impose order and rationality on his experiences often lead to more philosophical inquiries and existential musings. Through these vignettes, the narrative delves into themes of human perception, the nature of knowledge, and the interplay between the observer and the observed, all conveyed with a blend of humor, irony, and poignant insight.

In "The Island of the Day Before," a man finds himself stranded on a deserted ship in the South Pacific during the 17th century. As he struggles with his isolation and memories of his past life, he becomes obsessed with solving the problem of calculating longitude, a crucial challenge for navigators of his time. The narrative weaves intricate layers of history, science, and philosophy, exploring themes of knowledge, time, and the elusive nature of truth. The protagonist's reflections and adventures confront the limits of human understanding and the intersections of myth, science, and the power of storytelling.