Season 1

1. Natural Selection

British punk con artist Sarah Manning, living in Toronto, sees a woman who could be her twin, Beth Childs, kill herself by jumping in front of a train. Retrieving Beth's bag and finding that she was wealthy, Sarah decides to take her home and clean out her bank account. This would enable Sarah to collect her daughter Kira from her foster mother, Mrs. S. Sarah asks her foster brother Felix to identify Beth's body as Sarah's to buy time for her scheme and to throw her drug-dealer boyfriend Vic off her trail.

Sarah withdraws $75,000 from Beth's account but discovers, to her dismay, that Beth was a police detective suspended for shooting one Maggie Chen. Beth's police partner Art intercepts Sarah, who poses as Beth, and takes her to a hearing for Chen's death. Sarah ingests soap and vomits to stall the proceedings and try to escape. However, Art breaks into her car, taking the $75,000. After Sarah's funeral, with only Felix knowing she is still alive, Sarah is approached by Katja Obinger, another lookalike, from Germany. Katja is killed by a sniper, but Sarah manages to escape.

2. Instinct

Sarah is contacted by a woman who tells her to dispose of Katja's body and retrieve her briefcase. Art tells Sarah that he has the $75,000, and won't return it until she has testified at the inquest into the shooting and cleared her name. Art doesn't want it to be revealed that he put a cellphone in the dead Maggie Chen's hand so it would look like Beth had shot her in self-defence. Sarah poses as Katja to get the briefcase from the hotel. The briefcase contains evidence of more lookalikes, including soccer mom Alison Hendrix. Sarah gets Alison to set up a meeting, where she meets yet another lookalike: evolutionary developmental biology student Cosima Niehaus, the woman she spoke with on the phone.

3. Variation Under Nature

Alison tells Sarah that they are clones. In the briefcase, Sarah and Felix find that three of the clones Katja had found were already dead. Sarah goes to the police station to ask Art for the money back, only to be reinstated as a police detective and drafted to cover Katja's murder. Worried that her own fingerprints might be identical to Katja's, she deletes them from the record. Art and Sarah track down the killer, who shoots at them. Sarah saves Art's life, and then gives chase, only to find that the killer is another clone, a Ukrainian named Helena. Sarah stabs Helena, but Helena escapes. Art returns the money to Sarah, who learns that Beth had received it from Alison to help with her investigation into their situation. Sarah thus returns the money to Alison.

4. Effects of External Conditions

Sarah and Art continue looking for the killer, while Sarah tries to cover up that both killer and victim are her sister clones. The police believe that the killer is a fundamentalist Christian, while Cosima deduces that the group Helena is affiliated with believes the clones are abominations and must be killed. Helena keeps a close eye on Sarah and the case, putting Sarah's cover in jeopardy. Mrs. S allows her to visit Kira, but because Sarah is busy on the case, Felix asks Alison to impersonate Sarah - otherwise Mrs. S would never allow her to see Kira again.

Alison is taken aback when Kira instantly knows she is not Sarah. Helena lures Sarah to Maggie Chen's apartment, where it is revealed that Maggie worked for the same group as Helena: Beth had killed Maggie to protect the clones. Sarah allows Helena to escape before Art arrives, and later quits the police force. Helena collapses and is picked up by a man wearing the same symbol as her and Maggie.

5. Conditions of Existence

After waking up from a supposed dream where she was experimented on, Sarah spits out an electrode from her mouth. The clones soon learn that they have monitors, people in their lives whose purpose it is to observe them. Sarah suspects Beth's boyfriend Paul Dierden of being her monitor, while Alison suspects her husband Donnie. Meanwhile, Vic comes across a chance meeting with Alison, and discovers that Sarah faked her death. In response, Sarah pays Vic $20,000 to leave her alone: $15,000 to pay for his debts incurred by her theft and $5,000 to pay for his "pain and suffering" due to the loss of his finger over the unpaid debts.

Sarah and Felix spy on Paul's work place, but unknown to them, Paul follows Sarah walking Kira home from school. Paul also discovers that "Beth's" scar is gone. After calling Sarah to meet him at their home, Paul holds Sarah at gunpoint and demands an explanation, prompting Sarah to tell him that Beth is dead. When Sarah asks Paul why he is monitoring her, Paul admits that he does not know.

6. Variations Under Domestication

Alison purchases a nanny cam and finds that Donnie snuck out during the middle of the night. Paranoid, Alison ties him up, while also handling a neighborhood house party. Sarah impersonates Alison to question Donnie, who says he was covering up letters from a girl he dated when he and Alison were broken up. Paul follows Sarah and ultimately learns of their nature as clones. He also helps Sarah get rid of Vic when he continues to be a nuisance to her. Felix suspects Alison's neighbor, Aynsley Norris, of being the actual monitor. Meanwhile, Cosima is approached by Delphine Cormier, a fellow student. Cosima suspects Delphine of being her monitor, but goes with her to a lecture given by Dr. Aldous Leekie of the Dyad Institute, a proponent of "neolution" (using technology to advance human evolution ). Later, it is revealed that Delphine and Leekie are lovers.

7. Parts Developed in an Unusual Manner

Paul is questioned by his handler, Olivier Duval, regarding Beth's behavior. Though Paul tries to cover up the fact that Sarah is posing as Beth, the neolutionists discover the truth after checking the test results from experimenting on her. The neolutionists also believe Sarah to be the killer of the other clones. Meanwhile, Helena is nursed back to health by her trainer, Tomas, who orders her to kill Sarah. Although she appears reluctant to do so, she implores Sarah to reveal the names of other clones. Later, Sarah calls Helena to help free Paul who is imprisoned in Olivier's nightclub, called "Neolution." Paul and Sarah hide out in Felix's loft to avoid the neolutionists. The police continue to investigate Katja's body and are perplexed to find a DNA match between the body and the killer, and find the record of recently deceased Sarah Manning, who looks like Beth.

8. Entangled Bank

Paul blackmails Olivier into telling Leekie that there was only one clone who attacked, Helena. Leekie tells Paul he has a new job for him, then arranges for Olivier to be killed. Delphine tells Leekie that Cosima made a pass at her, and he presses her to find out how many clones Cosima has had contact with. Cosima and Delphine sleep together. Delphine then searches the apartment, finding the clones' names and informing Leekie, but neglecting to mention Kira. Art goes to the morgue to check "Sarah's" body, and this leads him to Felix.

He questions Felix and realizes it was he who made the anonymous call with information about Beth. Alison takes revenge on Aynsley, whom she suspects of being her monitor, by having sex with her husband, Chad. Aynsley finds out and confronts her, and they fight. Sarah introduces Alison to Mrs. S and tells her about the clones. While they are talking, Kira opens the front door to Helena and follows her out. Kira speaks kindly to Helena, who lets her go, but, hearing her mother call, Kira runs into the street and is hit by a car.

9. Unconscious Selection

Kira is taken to the hospital, where it appears that she has accelerated healing. Alison returns home to find that Aynsley is holding an intervention regarding her past behavior and addictions. Under Felix's encouragement, Alison turns the tables by scrutinizing Aynsley. Art finds CCTV footage of the train station where Beth committed suicide, and discovers that Sarah has been impersonating her. Cosima learns that Delphine leaked the names of the clones to Leekie and confronts her. Delphine reveals that she knows about Kira's existence but decided not to tell Leekie, knowing her life could come into danger should he know (Kira is the only known biological child of a clone).

Sarah meets with Leekie, who wants her to save Helena from the Proletheans, the extremists who indoctrinated her. Tomas punished Helena for defying orders, but she is later rescued by Sarah and turns against Tomas. Sarah returns to Mrs. S, who has reached her contacts in England, and meets Amelia, Sarah's birth mother. Amelia reveals that she gave her away and fled after realizing she was a surrogate mother to Dyad scientists. Furthermore, Amelia gave birth to twins; Helena is Sarah's twin sister.

10. Endless Forms Most Beautiful

As Amelia meets Helena, police arrive to arrest Sarah, while Helena escapes. Sarah is about to tell Art the whole story, but gets spirited away by Daniel Rosen, a Dyad operative. Sarah is brought to Rachel Duncan, a clone high in the Dyad organization, who offers Sarah and Kira protection. Leekie also offers the same to Alison and Cosima, as Cosima begins to display symptoms of a respiratory illness that Katja had. Alison confronts Aynsley a final time, ending with Alison watching Aynsley accidentally choke to death on her own scarf.

Amelia tells a woman pretending to be Sarah that Mrs. S is not who she says she is, but this "Sarah" is actually Helena, who stabs Amelia, blaming her for turning her over to the Proletheans. When Sarah finds her body, she shoots Helena. Alison signs Leekie's document, while Sarah goes to meet Rachel, planning to sign as well. However on the way, Cosima and Delphine decipher a code inside the clone genome: "this organism and derivative genetic material is restricted intellectual property." By phone, Cosima warns Sarah, who leaves Dyad without signing the contract. Donnie meets with Leekie, thus revealing he is Alison's monitor. Sarah returns to discover Mrs. S's home ransacked and Kira and Mrs. S gone.