"Darkly Dreaming Dexter" is a novel by Jeff Lindsay. Dexter Morgan is a blood analysis expert, who also happens to be a serial killer. Dexter's foster father, a police officer named Harry Morgan, taught Dexter how to kill in a way that will keep him from getting caught. Harry also taught Dexter to only kill people who deserve to die. As a result, Dexter only targets serial killers and child molesters. When, through his sister, Dexter gets involved in the case of a new serial killer who kills with an absence of blood evidence, Dexter finds his serial-killing soul mate. The only question is, will Dexter help his sister bring the killer to justice, or will he save this killer for his own brand of justice. Darkly Dreaming Dexter is an amusing twist on an old formula that will leave readers rethinking their own definition of justice.

Dexter works for the Metro-Dade Police Department. He is called to the scene of a murder by his sister, a police officer with vice. A prostitute has been murdered and Deborah wants to work the case so she can put herself in a position for a promotion to detective. When Dexter arrives on the scene, he discovers a murder that fascinates the killer inside of him. The victim is completely devoid of blood.

Deborah begs Dexter to help her solve this murder. Dexter is reluctant because he wants to find the killer and learn how he kills his victims, after which Dexter will then kill him. However, Dexter is aware that Harry, his foster father, would want him to help Deborah. Dexter agrees. While Deborah and Dexter begin their investigation, Detective LaGuerta, who is officially in charge of the case, begins her own. LaGuerta starts off on the wrong foot when she misinterprets some evidence and assumes the killer was interrupted during his latest kill. LaGuerta has her people concentrate on finding a witness, while Dexter has Deborah looking for a refrigerated truck. Dexter believes that rather than having been interrupted, the killer has grown bored with his routine. Dexter suggests the killer will kill again, quickly and bigger than before.

While Deborah looks for the refrigerator truck, Dexter goes on a date with his girlfriend. When Dexter was a child, his foster father discovered Dexter was unable to feel human emotions and had homicidal tendencies. Rather than seek counseling, Harry teaches Dexter how to indulge his needs without getting caught. Harry teaches Dexter about forensics as well as schooling him to only target people who deserve to be killed. Harry also teaches Dexter how to get along in society despite his inability to feel. Part of these lessons is the need for Dexter to seek female companionship. This is why Dexter has a girlfriend. Rita, his girlfriend, is a survivor of domestic violence and is not interested in sex. This makes Rita the perfect woman for Dexter, since he would never engage in sex.

At the end of Dexter's date, he sees a crime scene in the same area where the other victims were found. Dexter knows this must be another one of the Tamiami Trail killer's victims. Dexter stops to check it out and once again is fascinated by the completely bloodless way in which the killer kills. Rita is put off by Dexter's fascination with the killings; however, Dexter is so excited, he does not notice her distaste. Dexter drops Rita off at her house and in his excitement, he kisses her, which causes Rita to become agitated. Dexter watches her run into the house and is convinced that she will never want to see him again.

Over the next few days, Dexter begins to have dreams about the Tamiami Trail killer. It seems that Dexter dreams the details of the killer's future kills. These dreams cause the desire to kill in Dexter to come to life. Dexter takes a chance and kills a man he has been watching for a while. This kill is not as well organized as Dexter's crimes normally are, and he is nearly caught. The thrill leads Dexter to do something he would not usually do. Dexter has intimate relations with Rita. A few days later, Dexter goes for a drive in the middle of the night to clear his head, and he finds himself chasing a refrigerator truck like the one he imagines the killer would use. When Dexter loses the truck, he pulls to the side of the road only to have the truck reappear. The driver maneuvers the truck past Dexter and throws something out the window. It turns out to be a woman's head.

While Dexter submits to LaGuerta's questioning about the woman's head, they receive word that another body had been found. The body is at the ice rink of the local professional hockey team. Dexter once again sees the beauty in this kill, and he is more anxious than ever to find this killer. Later, Dexter discovers the killer has been in his house and left a message. Dexter believes the killer is talking directly to him through his victims. Understandably, Dexter is disappointed when it seems that LaGuerta has caught the killer. However, Dexter soon realizes that LaGuerta's prisoner is not the real killer.

Dexter warns Deborah that the killer will strike again. When the killer does kill again, the police find only three heads, each with an object that seems to be a message just for Dexter. Dexter tells Deborah that not only are these by the same killer, but suggests that the rest of the bodies are more than likely going to be found at the ice rink. Deborah and Dexter go to the ice rink and find the bodies arranged in a closet to spell out a message. While investigating this display, Deborah stumbles on an employee of the ice rink, who takes videos of the parking lot every night in order to post images of the hockey players on a fan website. This camera picks up an image of the killer, a killer who looks just like Dexter.

Deborah gives Dexter a few hours to figure out what is going on before she turns in the evidence. However, before Deborah can retrieve the images, she is kidnapped. Dexter goes in search of Deborah and is followed by Detective LaGuerta. LaGuerta has come to believe that Dexter has something to do with the murders. However, LaGuerta trusts Dexter enough to separate while they search the docks for a shipping container in which Dexter believes the killer is waiting. Dexter finds the killer first. The killer looks just like Dexter and turns out to be his brother. The killer wants Dexter to help him kill Deborah. Before Dexter can refuse, LaGuerta comes into the container. The killer stabs LaGuerta and continues to insist Dexter help kill Deborah. Dexter refuses, so he and his brother kill LaGuerta instead.