With the help of his newfound friend Brother Sam, Dexter wonders if there is light within him to counter the darkness, while the search for the Doomsday Killers takes him in a new direction; Batista and Quinn pay a visit to the university where Professor Gellar taught; due to departmental regulations following the shooting, Debra is forced to begin therapy.

Dexter is caught up in a very personal case that awakens the needs of his Dark Passenger; Debra feels overwhelmed by her new Lieutenant duties, made all the more complicated when she finds out Quinn slept with a witness in the Doomsday case.

Dexter finds himself teamed up with a fellow Dark Passenger when he takes a road trip to Nebraska to tie up some loose ends from the past; Debra deals with the complications of her promotion as she leads the Doomsday investigation.

Dexter uses lessons he learned from Brother Sam to follow up on some new leads in the Doomsday investigation; Debra butts heads with Captain LaGuerta over the case of a dead call girl.

Dexter receives help from an unexpected source while hunting the Doomsday Killers and staying one step ahead of Homicide; Debra discovers something new about herself in therapy.

Dexter tries to figure out the Doomsday Killers' next victim before it's too late; Debra has a strong reaction to a crime scene, which leads her to the realization that she leans too heavily on her brother.

Dexter finds that in order to catch the Doomsday Killers, he must create a macabre tableau of his own; Debra's battle with LaGuerta over the case of the dead call girl boils over, and her therapist makes an unnerving suggestion.

Dexter and Homicide race against a lunar eclipse to catch the Doomsday Killers before their final gruesome act; Debra struggles with a new emotional reality.

After witnessing her brother kill Travis Marshall, Debra attempts to reconcile with Dexter while struggling to cover up their involvement with the murder. Meanwhile, Detective Mike Anderson has an unfortunate run-in, Quinn and Batista begin to make amends, and LaGuerta finds evidence that causes her to re-think the closed Bay Harbor Butcher case.

Miami Metro sets out to unearth new evidence on a claim from a local convict that he has new information regarding a 15-year-old crime spree; Debra tries to cure Dexter of his killer tendencies.

Dexter tries to bring Debra on board with his new target. While the Ukrainian mob seeks revenge for the killing of one of their own, Quinn grows close to a dancer at their strip club.