While eating breakfast at Rita's house, Dexter is called to a crime scene, only to discover that it is the same salvage yard where he committed a double murder the previous night. He finds Valerie Castillo's (Valerie Dillman) body lying in the Airstream trailer where he killed her and her husband Jorge (José Zúñiga), despite having thrown the corpses into the ocean. He suspects that the "Ice Truck Killer"—a serial killer with whom Dexter has a personal connection—dived underwater to retrieve the woman's body and plant it in the trailer. In the yard, LaGuerta, Sgt. James Doakes (Erik King) and Dexter's sister Debra (Jennifer Carpenter) discover a young Cuban boy, Oscar (Cesar Flores), hiding in the trunk of a car who claims to have seen somebody take Valerie into the trailer. He begins to fear discovery, and experiences his first nightmare: Debra is a serial killer with an identical modus operandi to Dexter. As the investigation proceeds, he attempts to shoot down each of his colleagues' leads on the case. When Debra asks him to read through her report on the killer, profiling a man sharing many of Dexter's characteristics, he second-guesses her theory and she is hurt. Worried about coming under suspicion, he throws all of his knives overboard from his boat, but while looking at the blood drops from his victims, he notices that Valerie's slide has a happy face etched into the blood. He realizes that this is a hint from the Ice Truck Killer, and plants Jorge's fingerprints and a dry drop of Valerie's blood onto a knife, which he hides in a car at the salvage yard. He convinces Doakes to organize another search of the yard, where an officer (Denver Dowridge) finds the knife. After successfully framing Jorge for Valerie's murder, Dexter discovers that Oscar's description of the man who "saved" him from Valerie is in fact of Jesus, not Dexter.
Meanwhile, Rita dissuades her abusive husband Paul Bennett from attending their daughter Astor's (Christina Robinson) birthday party after he is released early from prison. Doakes takes Debra to dinner with his mother (Vernee Watson-Johnson) and sisters, while LaGuerta bonds with Oscar and considers adopting him until his uncle (Gabriel Salvador) arrives to take him home.
In flashbacks, a teenage Debra (Haley King) pleads with her father Harry (James Remar) to bring her on his and Dexter's (Devon Graye) hunting trips. When Harry forbids her from joining them, she steals his gun and practises shooting cans by herself. Later, Debra lashes out at Dexter in jealousy of the time that he spends alone with their father.
- Michael C. Hall: Dexter Morgan
- Julie Benz: Rita Bennett
- Jennifer Carpenter: Debra Morgan
- Erik King: James Doakes
- Lauren Velez: Maria LaGuerta
- David Zayas: Angel Batista
- James Remar: Harry Morgan
- C.S. Lee: Vince Masuka
- Valerie Dillman: Valerie Castillo
- Vernee Watson Johnson
- Andi Chapman
- Jose Zuniga: Jorge Castillo
- Christina Robinson: Astor Bennett
- Daniel Goldman: Cody Bennett
- Devon Graye: Teenage Dexter
- Haley King: Teenage Debra
- Cesar Flores: Oscar
- Damian T. Raven: Forbes
- Minerva Garcia: Mariel
- Kelli Kirkland Powers: Jess
- Lyn Alicia Henderson: Roni
- Demetrius Grosse: Alex Timmons
- Benton Jennings: Gene Marshall
- Julie Dolan: Cindy Landon
- Gabriel Salvador: Roberto
- Denver Dowridge: Police Cadet
- Gabriel Cordell: Sketch Artist
- Wing Armstrong: Medical Examiner Munoz
"Return to Sender" was generally praised by critics. Eric Goldman of IGN wrote that the episode's "most interesting facet" was its flashbacks to Dexter's youth from Debra's perspective, though he felt that the young Debra's hasty apology to Dexter after insulting him was "a slightly cheesy moment." He thought that LaGuerta's bonding with Oscar "was okay, but [the storyline] fell a little flat." TV Guide's Paula Paige "love[d]" the episode, saying that "It just keeps getting better and better," but was unsure "what Deb having dinner with the Doakes' family has to do with anything." Writing for TV Squad, Jonathan Toomey opened his review of "Return to Sender" by saying, "Dexter did it again. Wowed me. Amazed me. Edge of my seat for the whole hour." He was amused that Debra's profile of Valerie's murderer matched Dexter, but was uninterested in Rita's storyline with her husband.
Both Malene Arpe of the Toronto Star and Daniel Fienberg of Zap2it were disappointed with the episode's audio commentary featuring Jennifer Carpenter, Erik King, Lauren Vélez and David Zayas on the first season DVD.
- Dexter uses the alias of "Dr. Patrick Bateman" to buy narcotics, which is a reference to Patrick Bateman, a character similar to Dexter which appears in the novel American Psycho (which was also adapted to film).