At the same time Rinaldi is working with the FBI on a case involving serial killer John Jessup, who was convicted of murdering four prostitutes. Jessup, who was killed in a prison riot, had an admirer who wrote him complimentary, supportive letters signed 'Your Biggest Fan'. The admirer is now on a murder spree of his own, killing people responsible for Jessup's imprisonment and death. This seems to includes a prison guard, the judge, the prosecutor, jurors, the defense attorney, and Lyle Barnes, the FBI profiler who fingered Jessup. Barnes now suffers from night terrors and needs the help of a psychologist.
Local police are working with the FBI to protect potential victims and nab Jessup's admirer. They're hampered, however, because the killer seems to have inside information: he knows where the authorities are hiding potential victims, knows when they're about to question a witness, etc. Clearly, the investigation is compromised in some fashion.
While working on the cases Rinaldi can't seem to go a day without getting involved in a life-threatening situation such as chasing down a gunman, being run off the road by truck, putting himself in the path of a murderer, and so on. This seems unwise since Rinaldi apparently doesn't carry a weapon or possess martial arts skills. To me, he's somewhat of an unrealistic character - a sort of psychololgist/superhero - but he seems to be a decent enough guy. There's also a touch of romance in the story since Rinaldi gets together with Detective Eleanor Lowrey, a woman he's been attracted to for some time.
Eventually Rinaldi comes across information that's pivotal to both cases which leads to the book's climax. I thought this was a pretty good story with interesting characters and a nicely-constructed plot.