A woman wakes up in the morning to find that someone has picked her apartment’s supposedly impregnable door lock and rearranged personal items. Lincoln Rhyme and Amelia Sachs are brought in to investigate and soon learn that the sociopathic intruder, who calls himself "the Locksmith,” can break through any lock or security system ever devised. With more victims on the horizon, Rhyme, Sachs and their stable of associates must discover his true mission. Their hunt is interrupted when an internal investigation in the police force uncovers what seems to be a crucial mistake in one of Rhyme's previous cases. He is fired as a consultant for the NYPD and must risk jail if he investigates the Locksmith case in secret.