Dexter attends his 20-year high school reunion, hunting his next victim.
He begins contemplating the value of religion when Harrison has the chance to attend a Catholic pre-school.
This season features the Doomsday Killers (or DDK), Professor James Gellar and his student Travis Marshall, who seek to bring about the end of the world through killings based on the Book of Revelation.
They leave signs of the Apocalypse including the Alpha and Omega or the Four Horsemen as a cryptic tableau at each crime scene.
He takes Travis back to the church where Travis carried out his earlier murders, which Dexter has set up as a kill room.
The season's subplots include Angel's sister dating Louis Greene.
Initially believing Sam to be behind the first Doomsday Killer murder, Dexter decides to kill Sam but is quickly proven wrong and finds himself befriending him.
Travis marks Dexter as "the Beast" and tries to kill him in one of his tableaux, the Lake of Fire, but Dexter escapes and is saved by a passing migrant boat.
María La Guerta is promoted to Captain after blackmailing Deputy Chief Matthews, whose name was on a prostitute's ledger.
Vince Masuka is teaching a group of forensic science students, and after his first choice faints at a crime scene, he asks an attractive female student, Ryan Chambers, to become his intern.
Batista thinks he was passed over due to the feud between Matthews and La Guerta.
La Guerta continuously thwarts Debra's attempts to make a good first impression as the new lieutenant by giving her the wrong advice; however, Debra makes an independent decision to hire Mike Anderson as her replacement, against La Guerta's recommendation.