The Ice Truck Killer strikes again, leaving a bloody scene before Christmas at a holiday-themed park.
The homicide detectives are working at the latest crime scene of the Ice Truck Killer — body parts of the hooker with the amputated hand, who had been laid out under a Christmas tree with presents. Footage on a tape left at the scene reveals that the killing was done in a residential building with an impressive cold storage room.
Meanwhile, Debra is getting a lot closer to Rudy. His request for them to "get out of town" and talk has her puzzled. She asks Dexter to talk to Rudy, and he learns that Rudy is thinking of proposing to her. Dexter leaks this to Debra, who is quite excited by the news.
Dexter is still consumed by the desire to learn about his past. While researching newspaper articles from when he was a boy, he comes across the date October 3, or 103, the number laced throughout the crime scene. He then uncovers the truth: his mother was a police informant who was massacred. There is no mention of him in the news stories.
Dexter also has to deal with Rita's son, Cody, who has run away from home. He quickly realizes that Cody has gone to the park where he used to have court-ordered visits with his father. Dexter implores Rita to expose the children to their biological father, to let them see the kind of person he really is.
Dexter visits Angel, and sees teeth marks on the back of Angel’s head where he struck his assailant. Angel mentions that he must have hit his attacker on the lip. This strikes a chord in Dexter; Rudy has a split lip, and lied about it. Dexter is quick to investigate at Rudy's apartment, and picks up a blood sample from Rudy's trash. Analysis later reveals it to be a match for the blood found on Angel's shirt.
Lt. LaGuerta, meanwhile, receives word that she is being replaced. It is a direct consequence of making her boss look like a fool when she announced that the Ice Truck Killer was still at large, after the police assured the public that the killer had been caught.
Rita decides to follow Dexter's advice, and takes her kids to visit Paul in jail. This time she wants them to see him for who he is, not as "Disney Dad."
In the dénouement, alarm bells go off in Dexter's head, and he tries to stop his sister from seeing Rudy that evening. However, Rudy has lured Debra away with the promise of ‘only’ dinner. But Debra has made what may be a deadly mistake: Rudy proposes to her on a yacht, gives her drugged champagne, and ties her up with duct tape. Her disappearance leaves Dexter in a state of panic, and Rudy with a grin on his face.