Season synopsis
Contains spoilers for this season and earlier seasons.
Dexter begins the season married to Rita and trying to manage three children, including newborn Harrison, alongside the demands of his blood-spatter job and secret kills. Retired FBI investigator Frank Lundy returns to Miami to pursue the Trinity Killer, whose annual murder pattern has reached the city. Lundy and Debra are shot; Deb survives, but Lundy dies. Using Lundy’s notes, Dexter identifies Trinity as Arthur Mitchell, a church deacon with a family, and befriends him under the name Kyle Butler.
Dexter studies Arthur’s apparent ability to keep family life while killing, and lets the friendship influence how he handles Rita and the children. Arthur’s home is more violent than it first appears: he abuses Jonah and confines his daughter, while Dexter learns that Trinity’s ritual repeats deaths from Arthur’s childhood. Debra’s investigation into Lundy’s murder exposes Christine Hill as Arthur’s daughter and the shooter; after Arthur rejects her, she confesses and dies by suicide. Dexter discovers that Arthur abducts a boy before each murder cycle. Dexter rescues the captive, Scott Smith, as Arthur is about to bury him alive in cement.
Arthur eventually works out who Kyle Butler really is, and Dexter’s attempt to capture him is interrupted when a roadside collision gets Dexter detained. Arthur disappears before Dexter can return, so Dexter arranges for Rita to fly ahead to the Florida Keys while he follows. He later captures and kills Arthur, then returns home to find that Rita came back after forgetting her identification and that Arthur had killed her in the bathtub. Harrison survives beside her in the blood, repeating Dexter’s childhood scene. Debra also learns that Dexter and Brian Moser shared the same biological mother, without learning Dexter’s secret.