The book begins nine months after the end of Dexter by Design with the birth of Lily Anne Morgan, the daughter of Dexter and Rita Morgan. His daughter's birth has brought remarkable changes in Dexter; apart from feeling genuine love and emotions for the first time he also does not feel his Dark Passenger's compulsion to kill and vows to swear off his dark hobby in order to be a better father for his daughter. Soon after Dexter is called to a crime scene by his sister Deb, who is in the middle of a jurisdictional fight with the FBI who claim that a kidnapping has taken place. Dexter believes that the large quantity of blood found there was planted, and that the missing girl in question is faking her disappearance in order to get money from her parents. Dexter runs tests and discovers that the blood type does not match the missing girl, Samantha Aldovar. Deb and Dex go to the private school Samantha attends and talk to her principal, who at first is reluctant to divulge any information. This changes when the principal discovers that Tyler Spanos, a wild child and Samantha's friend, is also missing. Subsequent interviews with their friends indicate that they were both befriended by a young man with teeth filed down like fangs, and that only a few dentists in Miami offered such a service. Their prime suspect is narrowed down to Bobby Acosta, the son of Joe Acosta who is a city official and has powerful connections.
Dexter receives a surprise one day bringing Cody and Astor home from school; waiting for him is his brother Brian, whom Dexter last saw at the end of Darkly Dreaming Dexter. Brian quickly ingratiates himself with Dexter's family, who rapidly start to adore him much to Dexter's dismay. Dex soon receives another call from his sister, and arrive at a crime scene where someone was apparently cooked and eaten. Tests later determine that the gnawed bones found belong to Tyler Spanos after matching her DNA with that found at the first crime scene and medical records. One of the detectives working under Deb uses his contacts and come up with two Haitian men who swear that they saw Bobby Acosta leaving Tyler's car at a known Chop shop. Deb and Dex arrest a young man named Chapin after running from them to avoid questioning, but are forced to release him when a public defender shows up. Dexter, in a fit of overprotective fury over his daughter, stalks Chapin and kills him. Just before dying, Chapin admits to having taken part in eating Tyler Spanos. Things get worse when the remains of Deke, Deb's obnoxious partner, are found partially eaten. Rummaging through a nearby trash bin, Dexter finds Deke's blood sodden shirt and a chip from a local goth nightclub "Fang."
Dex and Deb force their way into the club, but shortly after finding Bobby Acosta they are thrown out by the club's irate manager. Deb resolves to wait until everyone leaves and makes Dexter break in to search for Samantha. Dexter breaks in and while searching around recalls that the manager of "Fang" was a prospective "playmate" due to the large number of migrant workers that the club went through who were never heard from again. Dexter eventually locates Samantha in a large refrigerator; while trying to get her out Samantha locks them inside and states that she doesn't want to go. Samantha then reveals that she desires to be eaten, and that she and Tyler shared the same fetish and volunteered to let the cannibals cook and eat them. Dex and Samantha are then taken to a trailer in the Everglades left only with a jug of water. While drinking the water Dexter and Samantha become euphoric and eventually have sex multiple times, despite the realization that the water was laced with ecstasy. Shortly afterward Deb and the Miami PD arrive (thanks to a tracer Deb's boyfriend Chutsky placed on the vans leaving the club) and arrest the cannibals, except for the club manager who was killed. Samantha, who is irate at being rescued, issues a thinly-veiled threat to Dexter that she will tell people he raped her as revenge for the police denying her her fantasy.
The next day Deb approaches Dexter and tells him that Samantha has run off again, and that she is going to talk to Bobby Acosta's parents in hopes that they will bring him in so she can find Samantha even though it will likely mean Bobby will avoid prison time. Bobby's father refuses to turn his son in, but his wife privately tells the two that he is at an abandoned amusement park that his father owns. Deb, Dex and Chutsky arrive at the park and begin searching it; eventually the three are caught and the leader of the cannibal "coven" is revealed to be Alana Acosta, Bobby's stepmother. Deb and Chutsky are taken away leaving Dexter to watch Alana cook pieces of a still-conscious Samantha. As Alana approaches Dexter to begin cutting and cooking him, one of Alana's guards guns her down as well as two other guards. Dexter's savior is revealed to be Brian, who had started working for the cannibals a few weeks prior. He cuts Dexter loose and helps him rescue Deb and Chutsky; while leaving they check on Samantha, who has since died of her wounds. Chutsky decides to leave Deb because he failed her and nearly got her killed; upon waking up on the way to the hospital Deb reveals that she is pregnant.
The book ends with Deb preparing to give birth despite Chutsky vanishing, and Dexter deciding that even though he now feels emotions like normal people, he can't stand by and let people be preyed upon when he can do something about it. He decides that the best he can do for his sister right now is to honor an earlier request of hers, and "take care" of Bobby Acosta.