...Eco's three protagonists work in the publishing
industry, where their efforts increasingly focus on
trashy books filled with mad occult speculations and
conspiracy theories. They have nothing but contempt
for the authors of these works, but out of sheer
boredom, they begin constructing their own half-
baked conspiracy theory—which they refer to as
the Plan with a capital P. At first, the Plan is merely
a private joke and idle entertainment, but increasingly
they work at it in earnest....

Excerpt from:
http://www.postmodernmystery.com/foucaults_pendulum.html

Foucault's Pendulum is divided into ten segments represented by the ten Sefiroth. The novel is full of esoteric references to the Kabbalah. The title of the book refers to an actual pendulum designed by the French physicist Léon Foucault to demonstrate the rotation of the earth, which has symbolic significance within the novel.