In the Beginning is the tenth of twelve episodes in Season Five and is the fifty-eighth of the series.
Dexter is carrying Harrison, thinking about how life repeats itself which is why he has to get Harrison out of Miami and Lumen out of the house to protect them from Jordan Chase. Sonya is put in charge of taking Harrison to his grandparents' house and he lets them go. Dexter brings Lumen to his apartment, but Liddy is watching them. Lumen says that Chase needs to be taken out now, but Dexter says that it will be difficult now that he knows about them. He shows her the picture of Emily Birch, the owner of the blood in Chase's vial. Lumen notes that she looks like her and the other girls in the barrels, only older. Dexter says that she might be the first victim and is going to talk to her soon.
LaGuerta is presenting the reopened Barrel Girls case evidence. Deb is back from her suspension in the file room and is allowed to search Cole Harmon's house. LaGuerta and Batista are still emotionally distant toward each other.
Deb and Quinn go to Chase's office looking for Cole Harmon, who he reveals has been missing for a couple of weeks. Chase denies knowing where Harmon is, but his anger escalates as the conversation goes on. He suggests that they talk to his attorney from now on. Quinn tells Deb that she was a bit harsh, but he is on her side. They go to Harmon's house, hoping the warrant has been issued.
Dexter and Lumen arrive at a house with high fences and barred windows, finding Emily Birch and asking about Jordan Chase. She denies knowing him, but then they show her the group photo Dexter found of The Group and Emily decides they should go. She also denies knowing the people in the photo and closes the door on them.
He goes to meet Deb at Harmon's house, where Deb points out a smear of blood on the wall. Dexter says that it is from where Lumen hit him when saving him from Harmon. Quinn brings to their attention 13 DVDs of Harmon's rapes and tortures. Dexter figures that Lumen is likely the 13th victim.
At the station, Deb rushes the bloodwork while Masuka puts up photos of the victims on the DVDs on the board. So far, he has yet to see disc 13. Dexter goes to his office and scratches a blank DVD and writes a '13' on it to switch it out with the real one, but Masuka stops him at the door. Deb has ordered him to follow chain of custody to prevent any evidence from getting out. But Dexter still manages to switch them after purposely knocking over the DVDs.
He returns home to Lumen, who has been trying to find info on Emily. Dexter brings her the DVD of what is supposed to be her rape and torture. Lumen asks if anyone watched it and Dexter says that he only watched enough to make sure it was the right one. Lumen cries, but is grateful that he was willing to risk everything to help her. Dexter says that they "both met each other at the right time". Later, Lumen watches her DVD, while Dexter listens from his bedroom.
The next day, Deb talks about how hard it is to watch the DVDs and Dexter tells her that people may be stronger than she thinks, referring to Lumen. But Deb says that maybe not. She mentions the 13th disc being too scratched to watch, but is glad to not have to watch another one. She has made a connection using the DVDs: Harmon has a ring matching the dentist from a previous homicide, concluding that they were likely friends and part of a club that raped and tortured women. She also notes that they, along with Boyd, are missing or dead. Someone is hunting them.
Lumen goes to see Emily Birch again and shows her footage from her DVD. She asks if this is what happened to her. Emily tell Lumen that 20 years ago, when she was a counselor in training, The Group including Jordan Chase (called Eugene Greer then) drugged her and she woke up in a cabin naked. Chase never touched her, but riled up the others to do things to her. Afterwards, they forced her to take the photo. Lumen asks about the last member and Emily names him as Alex Tilden.
Liddy arrives at the station and speaks to Dexter, listening as he takes a call from Lumen. As soon as Dexter leaves, Liddy forges Quinn's signature to get recording equipment from the police department.
Lumen meets Dexter and tells him about Emily Birch's confession about The Group but won't give him the last name unless he agrees to let her kill him. He warns that it will change her, but she says that she is already changed.
Quinn and Deb are already introducing themselves to Alex Tilden at his office, whose name appeared as a connection to Harmon. He denies knowing the other men thought to be suspects and nervously watches the officers leave. Deb mocks Quinn for his lax attitude with Tilden as they leave.
Dexter gives Lumen a gift for her first killing: a pair of black gloves. Liddy is listening to them plan Tilden's murder from a van using the equipment for the station. At Tilden's home, Dexter and Lumen look for evidence that he was involved. They find a gun and a box of jewelry, one of which Lumen recognizes as her own. Dexter allows her to take it. Together, they choose a kill room.
Emily Birch is talking to Chase about having talked to Lumen. She asks if he is in trouble, but he only says that Lumen has been "making waves". Chase tells Emily that no one could ever take her place and tells her how special she is to him without bothering to make eye contact with her.
Chase introduces his lawyers to Deb and Quinn. They have requested DNA to compare to the unidentified DNA on the girls. The lawyers try to get him out of it, but Chase agrees to give a buccal swab taken by Dexter. Chase goes into Dexter's office and calls Lumen toxic and dangerous on a self-destructive path. Dexter tells him that soon he will be cleared by DNA to allow him and Lumen to get to him.
Lumen and Dexter prepare to get Tilden with Dexter taken aback by her "kill outfit". Lumen practices her stab and Dexter watches her, only telling her to be careful. Liddy continues to listen and watch as they plan the murder.
Tilden talks to Chase, wondering if he should leave town. But Chase tells him that he has taken care of it and to act like everything is normal no matter what. Tilden agrees and is told to go home. Chase is waiting outside of Tilden's home, waiting for Dexter and Lumen to arrive, which they do. He calls Deb and tells her that Alex Tilden called his office looking for Cole Harmon about a flight. Deb thanks him and they leave for Tilden's home.
When Tilden arrives home, he is greeted to Lumen's DVD footage and she appears in front of him. He runs for his gun, but Dexter injects him and they take him to the kill room. Dexter takes a blood smear and Tilden asks for Lumen's name, showing him the pictures of the other victims instead. Deb and Quinn have arrived at his house and knock on the door, going inside when they find the door unlocked. They see a light under the door and open it, finding nothing. Dexter and Lumen are in the empty house next door. Tilden puts the blame on Chase and denies wanting to hurt Lumen or the other girls. He tries bargaining for his life, asking for a number. Lumen says, "13" and Dexter gags him. He watches Lumen stab him through the heart.
Outside, Deb finds a small partial footprint, noting that a woman was likely just there. She wonders if the thirteenth victim escaped and has been taking out the men as revenge.
Dexter and Lumen return home and Lumen kisses him. After they have sex, Dexter thinks to himself that in her eyes, he is not a monster.
- A picture of Chris Vance can be seen as the character James Whistler on Prison Break.