The novel's protagonist, Dexter Morgan, works for the Miami-Dade Police Department as a forensicblood spatter analyst. In his spare time, Dexter is a serial killer who kills murderers, rapists, and other undesirables he believes that the legal system has failed to stop. Dexter's murders are directed by an inner voice he refers to as the "Dark Passenger", which keeps prodding Dexter to satisfy his homicidal urges on a regular basis. Once he has done so, the voice is placated for a while, but it always eventually comes back. Flashbacks reveal that his foster father, an esteemed police detective named Harry Morgan, recognized early on that Dexter was a violent sociopath with an innate need to kill, and taught him how to kill people who had gotten away with murder as a way to channel his homicidal urges in a "positive" direction. Harry also taught the boy to be a careful and meticulous killer, to leave no clues, and to be absolutely sure his victims were guilty before killing them. Dexter calls these rules "the Code of Harry." Dexter succeeds in managing his double life until he investigates the "Tamiami Slasher," who has murdered three prostitutes. Dexter's adoptive sister, Deborah, who is also on the force and wants to be promoted to Homicide — and knowing that her brother has eerie "hunches" — asks him for help in solving the case. Dexter eventually decides to help Deborah due to his moral code, but still feels a compelling pull to the killer due to similar desires. After a lucid dream, Dexter drives around Miami and spots a refrigerated truck. When he follows the truck, the killer throws a severed head in his car. The killer begins sending messages to Dexter, who finds the crimes fascinating. Dexter is torn between helping Deborah and allowing the killer to continue his spree. Meanwhile due to his strange dreams, Dexter wants to kill somebody so he follows a man whom he suspects of raping and killing five teenage girls and kills that man after confirming that the man did those crimes. The killer kidnaps Deborah and brings Dexter to the scene, revealing that his sister is being held in the same shipping container Dexter's mother Laura was held in. The killer is Dexter's biological brother, Brian, who was separated from Dexter after their mother's murder at the hands of a drug dealer. As Deborah's spiteful colleague, Migdia LaGuerta, arrives on the scene, Brian is disappointed that Dexter refuses to kill Deborah. Dexter allows Brian to kill LaGuerta and helps him get away out of familial loyalty. In the epilogue, Dexter stands at LaGuerta's funeral and feels sad, but cannot bring himself to cry.
Award
Darkly Dreaming Dexter won the 2005 Dilys Award presented by the Independent Mystery Booksellers Association.
The novel is the basis for a TV series on the cable network Showtime. Whereas the first season largely followed the plot of the original book; subsequent seasons featured original storylines not directly based on subsequent "Dexter" novels. Darkly Dreaming Dexter was also featured on an episode of Booked, a Canadian television series that investigates crime fiction novels through the eyes of real forensic science experts.
In Episode 2 of the fifth season of the TV series Billions, Charles “Chuck” Rhoades, is in a session with a therapist. When prompted by the therapist to choose a 'wise person', Rhoades, an avid reader of crime fiction, nominates Dexter's father, Harry Morgan, for guidance.
