The story is about two strangers who become dangerously close after witnessing a deadly accident.
The film and the book differ a great deal, the most obvious change being the renaming of Joe Rose (Craig)'s love interest, Clarissa Mellon, to Claire (Morton), and the change in her position from common-law wife to girlfriend. The character is now a sculptor instead of the Keats scholar she was in the novel. Joe's profession has also changed from science writer to college professor, and Jed Parry (Ifans) is no longer living a life of comfort on his inherited wealth.
Several key scenes from the novel do not appear in the film, and in their place are new scenes devised by the screenwriter.
The stalking of Joe is quite a mystery in the novel - it is not clear at first whether or not it is simply within Joe's mind, as the reader understands that Joe is still in shock from the balloon accident. In the film the stalking is portrayed as obvious reality throughout.
Joe is driven to the boundaries of his sanity in the novel through the many possessive letters posted by Jed, which help the reader to understand Jed's state of mind. These are not included in the film.
There is a scene in the novel where Jed arranges for Joe to be shot while celebrating Clarissa's birthday at a restaurant. The wrong man is shot by the hit-men; this is the point in the novel where Joe grasps Jed's potential for violence. This scene is not included in the film; it instead appears in the film that Joe is the violent one, appearing in Jed's flat with a baseball bat.
In the climactic scene of the novel, Jed does not stab Clarissa or share a kiss with Joe. Jed instead moves from threatening Clarissa with a knife to slitting his own throat, which is only stopped by Joe shooting him in the elbow with the gun he had obtained earlier in the novel.