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Dexter uses his 20 year high school reunion as a chance to hunt the prom king. It's going fine until he is called to the scene of a vicious faith-based murder leaving him with questions about what he wants to pass on to Harrison. Debra who is finally in a good place with her relationship with Quinn and her job becomes an unexpected hero.
In order to solve the mysterious murders, Homicide asks for the help of Brother Sam, who Dexter knows used to be a serial killer. Debra has two proposals, one personal and one job-related.
Dexter faces a serial killer from his past; Debra has problems with her new position and Travis with Gellar think of their next move.
The team investigates murders that have a religious undertone to them.
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.moreless
Dexter investigates who shot Brother Sam. Meanwhile, Homicide interrogates the woman who Quinn slept with who is involved in the Doomsday Killer case.
Miami Metro gets closer to Gellar and Travis thanks to the new intern. Meanwhile Dexter deals with faces from the past. Travis tries to move on from Gellar.
Dexter, using what he's learned from Brother Sam, comes a step closer to the Doomsday killers, specifically Travis; Deb and LaGuerta go head-to-head after a call girl ends up dead.
Dexter tries to stay one step ahead of the Homicide Division concerning the Doomsday Killers, while he finds help from an unlikely source; Debra uncovers something unexpected about herself during her therapy sessions.
The mirror-match continues. Travis enlists some new acolytes. Miami Metro gets two new breaks. Dexter finds himself in hot water, confused about what to do with Wormwood.
Danger arrives to Miami Metro. Dexter continues to try and lure Travis in while Deb continues to see her therapist.
In the season finale, religious zealot Travis (Colin Hanks) continues his plan to bring about the end of the world, leading him to kidnapping Dexter's (Michael C. Hall) son, resulting in a climatic and bloody battle between the two of them on a boat.