Walter Sparrow (JIM CARREY) is a generally happy family man -- with wife Agatha (VIRGINIA MADSEN) they have a teenage son Robin (LOGAN LERMAN) -- who's rather bored with his job working for the Department of Animal Control. His attempts to capture a loose dog by the name of Ned, however, will forever change his life. Since he's late to meet Agatha, she ducks into a bookstore where she picks up the short novel, "The Number 23," a story of obsession about that particular integer and the way it repeatedly shows up and affects the story's characters.
She reads but doesn't think much of it, handing it off to Walter who has a much different reaction. And that's not only because he also starts seeing many things -- his driver's license and social security numbers, for instance -- that add up to or are somehow related to that number, but also due to him finding similarities between his life and that of the book's protagonist.
He's Fingerling (JIM CARREY), a seedy gumshoe who's having a fling with the seductive Fabrizia (VIRGINIA MADSEN) who's into faux rough sex. When not tying her up, he's out trying to stop Isabel, a.k.a. the Suicide Blonde (LYNN COLLINS) from living up to her name. It turns out her obsession with the number 23 -- that he discusses with Dr. Miles Phoenix (DANNY HUSTON) -- eventually segues over to Fingerling, eventually creating an obsession that threatens to destroy him just like all who've preceded him.
Back in the real world, Walter similarly becomes obsessed with the number, eventually leading to Agatha's teacher friend, Isaac French (DANNY HUSTON), trying to debunk myths about its believed supernatural significance. With repeated encounters with Ned leading to the gravesite of 23-year-old Laura Tollins (RHONA MITRA) and the man convicted of killing her, Kyle Finch (MARK PELLEGRINO), Walter tries to find out what's really occurring, an answer that will have repercussions that will jolt his world and existence in it.