Dexter finds himself not "on his game" after the death of his brother, the ice truck killer. He fails and shows mercy and then fails to prepare his victim after that. In the meantime his sister returns to work even though she still has some signs of mental trauma from her brush with death. A dramatic find at sea by treasure hunters triggers different feelings from himself and his sister.
The discovery of the dumping ground off the coast brings in a renowned FBI agent for solving practically impossible cases. Dexter continues to struggle but finally comes to terms with the death of his brother. Dexter reveals to Rita the truth about Paul's return to the prison system.
Dexter's sudden inability to deceive has him pursuing a slick liar as his next victim, while Rita believes he's concealing a drug problem and forces him into a 12-step program where he meets Lila, a seductive new woman.
While pretending to be a drug addict, Dexter gets an N.A. sponsor to please Rita. But when the sponsor turns out to be a beautiful woman who may have more in common with Dexter, Rita is not happy. Things get worse for Rita when her estranged mother comes for a visit. Meanwhile, Dexter must find a way to lead the all-too-smart FBI Agent Lundy, now working with Miami Metro PD, off his trail.
Dexter learns that one of the men who killed his mother is still alive, and decides to confront him with Lila so that his addiction does not get the best of him.
A copy-cat killer inspired by the Bay Harbor Butcher begins to stalk Miami criminals, much to Dexter's chagrin. As Dexter tries to stop the fake, he tries to recover a video that might expose him for who he really is.
The Bay Harbor Butcher sends the Special Task Force a written letter, making them all on edge. Continuing with his plan, Dexter tries to be more "pro-active" in keeping Lundy away from him, but things become complicated when Lila begins to take a toll on Dexter's personal life.
After there is a fire at Lila's house, Dexter finds that she has become more attached to him than he's comfortable with. Meanwhile, when "recovery" doesn't work for the man who killed his mother, Dexter pays the man a visit. At the station, Deb and Lundy find a clue that makes them suspicious that the Bay Harbor Butcher might be closer to them than they think.
As Dexter tries to break away from Lila, he finds that she's not ready to let him loose. Meanwhile, Lundy's search for the Bay Harbor Butcher continues, making Dexter nervous. However, Dexter's worst enemy discovers his secret.
Dexter gains the upper hand when Doakes follows him to his latest kill. But when Doakes reveals a secret about Harry, Dexter's world falls apart. Meanwhile, Rita begins to let Dexter back into her life and the Miami PD narrows in on the Bay Harbor Butcher.
Dexter begins to dodge bullets, both figurative and literally. Now, Dexter must decide his own fate, as well as those he cares about most. Lila tries to blackmail Dexter into becoming her lover, but makes a discovery that changes their lives forever.
The Bay Harbor Butcher case is closed, but Dexter continues to deal with his issues with Lila and Doakes.