The episode begins with Dexter sitting in on his first Narcotics Anonymous meeting. He is bored by the repetitive stories of others, but a dark-haired woman catches his eye briefly. He leaves the meeting early. At Rita's house, Rita gets upset when he doesn't come back with a newcomer's chip. Trying to appease Rita, Dexter promises to stay for the entire meeting next time.
The next day at work, Dexter walks in to find a mob of people all clamoring to report their missing loved ones as possible victims of the Bay Harbor Butcher. In the meantime, Special Agent Lundy announces his selections for his task force, including Masuka, Angel and Deb (to her great surprise).
Later that day, Dexter is checking up on his next intended victim: Roger Hicks, a used car salesman. As Dexter tries to collect a sample of Hicks' DNA to confirm his involvement in several murders, Hicks tries to sell Dexter a minivan. To Dexter's own surprise, he ends up buying the car.
Back at the station, Deb starts interviewing the mob, but finds the work emotionally draining. She asks Lundy to take her off the task force. Masuka brings Dexter into the field morgue to help him out, giving Dexter a chance to see his entire "body of work" laid out.
That night, Dexter is able to slip away from Doakes and attends another NA meeting. When he tries to claim his newcomer's chip, he is forced to speak in front of the group. He improvises a story about being a recovering heroin addict. Lila, the mysterious dark-haired woman from the previous night, easily sees through his performance. As they begin to talk, she demonstrates an uncanny understanding of Dexter's true "addiction", or at least the manner in which it controls him. Unnerved by Lila's recognition of his "dark passenger", Dexter leaves hurriedly and decides it is too dangerous to come back to the meetings. At Rita's house, Dexter tells Rita that he won't be going back to the NA meetings. Rita promptly kicks him out.
The next day, Dexter is back at Hicks' car lot, looking for more proof of Hicks' inner monster. In Hicks' office, he finds a DNA sample on a discarded cigarette butt. He runs into a pretty brunette who he recognizes as Hicks' next target; Dexter knows he must act quickly, but must also keep a low profile due to the BHB manhunt.
At the station, Pasquale is becoming increasingly distracted by her suspicions that her fiancé is cheating on her. In the lab, Dexter is able to use Hicks' DNA to put him at the scene of previous murders. Back in the bullpen, Deb nearly has an emotional breakdown when she interviews a distraught mother looking for her missing daughter. She storms into Lundy's office, demanding again to be taken off the task force. Lundy refuses, explaining that Deb's experience with the Ice Truck Killer may give her potentially valuable insights into catching the Bay Harbor Butcher.
Captain Matthews drops by the station to speak with LaGuerta about Pasquale's "erratic" behavior. When he asks LaGuerta for her honest opinion, LaGuerta fires back that Pasquale is fine, and that she's simply being treated unfairly by a sexist, male-dominated department.
Dexter drops by the field morgue and finds Lundy there. They briefly discuss the psychology of the BHB, and Lundy reveals his belief that killing is justified when it is done to save innocent lives. Dexter takes this to heart; he slashes Doakes' tires and heads off to capture and kill Hicks.
After subduing Hicks in his own car lot, Dexter sets up a kill room in the home of one of Hicks' previous victims. As he confronts Hicks with his crimes, Dexter is amazed and impressed by his ability to deny the accusations and lie effortlessly. In a moment of complete honesty, Dexter discusses his problems with Rita and comes to the realization that he cares deeply for her (and this is the reason he bought the minivan). He kills Hicks.
Dexter returns to the NA meeting, revealing his real name to the group. He speaks from the heart about his struggle with his "dark passenger", as well as the uncertainty he feels, now that he's come to realize he cares about others. As the meeting draws to a close, Doakes shows up and confronts Dexter. He mistakenly assumes that Dexter's dark secret is a narcotics addiction and seems to be satisfied with this explanation for the time being.
At the station, Deb interviews the relative of another missing person. When she finds that the missing person has a prison record, she begins to see a pattern and runs it by Lundy. Lundy agrees, and they begin checking the DNA of the BHB's victims against a criminal database.
Dexter drops by Rita's place to tell her the good news about his breakthrough at NA. Rita happily embraces him. The next morning, as Rita drops Dexter off at work, she gets her first look at Lila. As the camera zooms in on Rita's face, we see that she is obviously concerned that Dexter's new sponsor is an attractive woman.