The main character, named Dexter Morgan, works at the Miami Police Department as a forensic expert specializing in blood spatter. When he’s not working, his main hobby is killing people. That’s what his inner voice tells him—what he calls the "Dark Passenger."
But Dexter isn’t just a killer sociopath. According to flashbacks, in early childhood his adoptive father, a police officer named Harry Morgan, noticed his sociopathic tendencies and taught his adopted son to control his irresistible drive to kill. Harry showed him how to hide bodies, get rid of evidence, and, most importantly—how not to draw attention to himself. These rules are written into what is known as (by Dexter himself) "Harry’s code," with which he decides who gets to live and who does not. According to the code, one may kill only those people who deserve death—and for whom there must be substantial evidence of guilt. Usually he gets that evidence himself. One item in the code is socially acceptable behavior, which Dexter does perfectly. Dexter also can’t feel human emotions, as he believes—partly for the same reason that gave him the "Dark Passenger." Therefore he learned to imitate almost all emotions. Human mannerisms seem strange to him; he doesn’t feel any sexual desire.
Despite this, he has a girlfriend—Rita, the mother of two children named Astor and Cody. He feels sympathy for the children, especially for this pair.