The novel’s protagonist, Dexter Morgan, works for the Miami-Dade Police Department as a forensic blood spatter pattern analyst. In his spare time, Dexter is a serial killer who kills murderers, rapists, and other undesirables he believes have escaped, are escaping, or will ultimately escape justice.
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, a 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."