Plot
As the police arrive at Dexter's house, he is obviously in shock and, either because he feels guilty that his relationship with the Trinity Killer, Arthur Mitchell, caused Rita's death or because he is answering a question ("Sir, did you say that you called this in?") asked by one of the police officers, and admits "it was me." Joey Quinn is already suspicious about the circumstances surrounding Rita's death, considering it did not follow Trinity's modus operandi. He is also suspicious of Dexter's unemotional manner after the incident. Astor takes Rita's death particularly hard, and blames Dexter for it. Unable to reconcile, Astor decides that she wants to live with her grandparents in Orlando, Florida, and she and Cody leave Miami.
Miami Metro begins investigating a severed head left in a Venezuelan neighborhood, and also find several related cases. The suspect is quickly nicknamed the Santa Muerte Killer. The FBI, unable to find Arthur Mitchell, follows its only other lead, Kyle Butler (Dexter's alias when interacting with the Mitchell family). Quinn recognizes the similarities between sketches of Kyle Butler and Dexter. Dexter finds a Department of Sanitation worker, Boyd Fowler, who is responsible for the deaths of several women. Dexter hires a nanny named Sonya to care for Harrison. Dexter eventually kills Fowler, but the crime is witnessed by Fowler's next victim, Lumen Pierce, who he has in captivity.
Dexter tries to care for Lumen, but she is understandably suspicious of his motives, asking Dexter if he is going to "sell her". Quinn tracks down the Mitchells, who are now in witness protection. He approaches Jonah Mitchell at a small convenience store and shows him a picture of Dexter, asking if it is Kyle Butler, but an undercover FBI agent interrupts before Jonah can answer. Quinn is suspended without pay by LaGuerta for disobeying her orders as she continues to defend Dexter. With Dexter's help, Deb closes in on the Santa Muerte killer. Debra lets him escape during a hostage standoff in order to save the hostage's life.
Lumen tells Dexter that she was attacked by a group of men, not merely Fowler on his own. Lumen asks Dexter to help her seek revenge on these men, but he initially refuses. After Lumen continues on her own and mistakenly targets the wrong suspect, Dexter teaches her the importance of knowing a person is guilty. Dexter accompanies Lumen to the airport and believes she has left Miami. Instead, Lumen remains behind, hunts down and shoots one of her attackers, and out of desperation asks Dexter to help her clean up the crime scene. Dexter reluctantly agrees, and they finish moments before homicide police locate the crime scene. Lumen later reveals to Dexter that killing one of her attackers brought her a sense of peace. She tearfully recognizes that it will not last and that she will have to find (and kill) the others to experience that peace again. Dexter recognizes this as being her own Dark Passenger. He decides to help her, partly to atone for his earlier inability to save Rita.
Meanwhile, LaGuerta and Angel Batista make their marriage public but are having marital issues, and Quinn and Debra become romantically involved. When Angel gets involved in a bar fight which gets him in trouble with the Internal Affairs Department (IAD), LaGuerta saves him by helping IAD set up a sting on another cop who is under suspicion, Stan Liddy. Liddy develops a friendship with Quinn, their common bond being they were both "betrayed" by LaGuerta, and Quinn pays Liddy to investigate Dexter. Debra remains unaware that Quinn suspects her brother is Kyle Butler.
Dexter and Lumen hunt the other people responsible for torturing her, including their leader, motivational speaker Jordan Chase, and people associated with him. As Quinn's relationship with Deb deepens, he tries to stop Liddy's investigation, but by this point Liddy has taken pictures of Dexter and Lumen on Dexter's boat disposing of large plastic bags and video of them practicing for a kill, and is determined to continue. Having lost his job and become convinced Dexter is a criminal, Liddy captures Dexter and calls Quinn to tell him to come to his location. A struggle ensues and Dexter kills Liddy and destroys Liddy's surveillance footage. Dexter learns Lumen has been kidnapped by Jordan Chase and leaves the crime scene to try to find her. Quinn, having responded to a call from Liddy, finds Liddy's van locked and apparently empty; a drop of Liddy's blood falls on his shoe, unnoticed. Dexter returns home to collect his tools to attack Jordan. He is surprised to see Astor and Cody who want to have their baby brother Harrison's first birthday party in Miami and stay with him the coming summer.
Before Dexter can confront Jordan, he is called away to Liddy's crime scene, where the police suspect Quinn's involvement in Liddy's death after noticing the blood drop that fell on his shoe earlier. Quinn initially complies but later refuses to answer questions. Later, Dexter learns of Jordan's (and Lumen's) whereabouts. The two briefly struggle. Dexter overpowers Jordan, and then allows Lumen to kill Jordan. After the kill, Deb discovers the two of them, though they are behind translucent plastic and she is not able to see their identities. Understanding that one of the two figures must be an escaped victim, Deb sympathizes and retreats so they can escape. After Jordan's death, Lumen no longer feels the need to kill and tearfully admits she needs to move on, leaving Dexter distraught.
Quinn talks with Dexter at Harrison's birthday party, where Quinn thanks Dexter for exonerating him, as Dexter has faked a blood test to clear Quinn. Quinn and Deb appear to reconcile, as do Maria and Angel. Blowing out Harrison's birthday candle, Dexter wonders if there is hope for him — to have a genuine relationship, to be human — but he doubts it.
