[7] During this time, Debra struggles to recover after surviving the Ice Truck Killer's attempt to murder her, and Rita sends Dexter to Narcotics Anonymous meetings when he tells her that he has an "addiction".
[7] Sergeant James Doakes (Erik King) stalks Dexter, suspecting that he is connected with the Ice Truck Killer murders.
[8] Three new characters are introduced:[9] Keith Carradine appears as Special Agent Frank Lundy, an FBI agent who heads the Bay Harbor Butcher investigation;[10] JoBeth Williams as Rita's mother Gail; and Jaime Murray as Lila Tournay, Dexter's Narcotics Anonymous sponsor.
[11] The season was praised by critics as "one of the best shows on TV this decade" by the Chicago Sun-Times,[12] while Variety considers Hall's portrayal of the title character as a "towering achievement, one that eclipses the show's other shortcomings and rough patches";[13] the aggregate site Metacritic scored the season at 85 out of 100 based on 11 reviews.
James Doakes monitoring his activities and his sister Debra now living with him as she recovers from her traumatic experiences concerning Brian, the Ice Truck Killer.
Rita doubts Dexter's reliability and honesty after finding evidence that he set up her husband Paul to be returned to prison.
Rita incorrectly concludes that Dexter is, like Paul, a drug addict and that this explains his occasional absences and odd behavior.
Divers accidentally stumble upon Dexter's underwater burial ground, discovering the many bags containing the body parts of his victims.
When it's revealed that each victim was a criminal and killer, some members of the public openly support the Bay Harbor Butcher; the case even inspires the creation of a knife-wielding comic book superhero "The Dark Defender."
To ensure he's not identified as the Bay Harbor Butcher, Dexter finds a new dumping area with current that leads to the Atlantic Ocean.
Dexter later learns that his biological mother died because she was a criminal working as a confidential informant for Harry and had an affair with him.
Lieutenant LaGuerta vouches for the innocence of her former partner, but Lundy refuses to consider her evidence after he learns that she didn't report previous contact with Doakes during the period he was a fugitive, because of their personal relationship.
Dexter subdues Doakes and locks him in a makeshift cell within the cabin, admitting to the sergeant that he is indeed the Bay Harbor Butcher.
Deciding she now understands Dexter and must help him, Lila leaves Doakes imprisoned and then lights the cabin's gas stove and opens a propane tank.
At the same time, Debra is on her way to leaving Miami with Lundy rather than letting their relationship end, but then misses the flight when she learns that the children are in danger and Dexter needs her.
The season concludes with Dexter tracking down Lila to Paris and killing her, avenging Doakes and ensuring that no one alive knows his secret life as a serial killer.