Season 5 opens with Dexter dealing with the aftermath of Rita's murder and making a decision that will impact many of those in his circle of friends, relatives and coworkers. Meanwhile, Quinn concludes that Rita's death doesn't match Trinity's MO, and steps up to help Debra in a time of trouble.
On the heels of Rita's murder, Dexter tries to get the family back into a familiar routine, but a frightening discovery may lead to his next quarry. Meanwhile, a brutal murder in a Venezuelan neighborhood has the squad scrambling; and Debra, who's opened her home to Dexter and the children, finds herself without adequate accommodations, and turns to Quinn for help.
Dexter hires a nanny to help free up time so he can focus more on stalking murder suspects. Meanwhile, Debra finds her rookie partner to be intolerable, and the newbie disagrees with her theories on a double homicide; and Quinn has a theory that Trinity is not responsible for Rita's murder.
Dexter catches a mysterious woman who may have witnessed him in the act of killing a killer; Deb has a frightening run-in with a murder suspect in the Santa Muerte homicides; and Quinn uncovers more striking similarities between Kyle Butler and Dexter Morgan.
Dexter must deal with his unwanted conspirator while wondering if Rita's death and Harrison's exposure to it may have had a subconscious impact on the infant. Meanwhile, Quinn's suspension leaves Deb to work the Santa Muerte homicides alone, but frees him up to look further into Dexter's background and possible involvement in the Trinity Killer case.
Striving to bring some normalcy back into his shattered life, Dexter figures the answer lies in finding a new serial killer to bring down. His quest gets thrown off course, however, when his troubled new acquaintance requires his help. Meanwhile, Batista uncovers a revealing new lead in the Santa Muerta homicides, and Quinn gets a tip about Dexter but isn't sure he can trust his source.
By aiding Lumen (Julia Stiles), Dexter puts himself on a collision course with Debra and the homicide division, and things get worse when he's brought in to work a horrific crime scene. Elsewhere, Debra's investigation into the Santa Muerte murders leads to a deadly confrontation with the killers.
Dexter and Lumen (Julia Stiles) take the opportunity to track a vicious murderer while attending a self-help seminar. Meanwhile, fallout from the Santa Muerte homicides puts Debra in hot water and places Batista in a difficult situation.
Dexter and Lumen (Julia Stiles) are on the hunt for a new victim but get sidetracked by the arrival of a surprise visitor. Meanwhile, as a result of fallout from the Santa Muerte case, Debra gets reassigned to the file room, where she is still able to dig up new evidence in the Barrel Girl investigation.
While on the trail of Lumen's former attackers, Dexter stumbles onto a possible ally, but when the homicide unit finds evidence linked to one of Dexter and Lumen's earlier victims, they are forced to throttle back. Meanwhile, Debra identifies two more suspects in the Barrel Girls investigation.
Somebody outside of the department is showing a vested interest in Dexter and Lumen's clandestine activities, forcing him to engage in his special kind of damage control; Debra may be zeroing in on the whereabouts of the missing Barrel Girl suspects; Quinn is reticent about getting more deeply involved in a recent homicide.
On the fifth-season finale, Dexter smells a trap being set for him and Lumen (Julia Stiles) and pulls out all stops in an effort to avoid getting caught in it; personal feelings get in the way of Debra's investigation into the Barrel Girls; and Quinn gets in a bind from which only Dexter can extricate him.