In a quiet suburban home, Mike Halloran and his wife Beth hear something in the living room. Mike goes to investigate while Beth calls the police. Two burglars break in, clubbing Mike down. However, something attacks all of them as they scream in terror.
Torchwood arrives to investigate as the two burglars are taken away. One burglar was stabbed and killed with a long narrow blade but there's no weapon present. The other was thrown out the window and is in critical condition. Tosh doesn't believe the husband or wife could have done it. Gwen and Owen talk to the Hallorans, who have nothing useful to say. Owen determines that there's no trace of blood on their hands. Owen thinks Beth attacked the burglars but Gwen isn't convinced. However, Owen can't explain how she did it and left no evidence. Gwen thinks Mike did it but Owen notes that there's no weapon. Jack calls to get an update and tell them to watch the surviving burglar. Owen has Gwen do it and goes to get some coffee. As he goes, the power flickers briefly.
Gwen sits at the burglar's bedside and starts to doze off. The burglar wakes up and asks Gwen to keep Beth away from him before he dies.
Jack takes Beth to Torchwood and demands answers. Beth insists she never touched the burglars. The power flickers again and Gwen goes a softer approach. Beth still insists that she has no idea what happened to them, but it wasn't her. Tosh determines there was an electromagnetic buildup around Beth but can't pinpoint if she caused it. Jack orders tests on Beth, and Gwen suggests that she cooperate. Owen tries to take a blood sample but the needle keeps breaking. He soon discovers that his skin is tough enough to break a scalpel. When asked, Beth confirms that she's never been to a hospital or doctor and never been sick. She insists she's from Earth and that there's no such thing as aliens. Jack takes her to see the Weevil, which back nervously way from her.
Jack insists on using a mind probe on Beth to get at the truth. Despite the pain, Beth insists that she's human. There's another electromagnetic buildup but Jack orders them to continue. The pulse goes off the scale and Beth finally passes out. As she does, her right arm transforms into a mechanical device. Tosh confirms that Beth couldn't have known about it. The woman starts speaking in an alien language and Jack confirms all she's saying is her name, rank, and serial number, and Jack says he knows why she's there and who she is. Once they switch off the probe, Beth reverts to normal with no memory of what happened.
Jack explains to the team that Beth is a sleeper agent for an alien race codenamed Cell 114. They infiltrate planets and gather intelligence until they take over. They give their agents false memories to blend in, but her real self took over to preserve itself and kill the burglars. The implant gathers information and generates a force field. Tosh taps the data files in the implant and realizes that it's gathered masses of information, including data on Torchwood.'
Jack decides to inform Beth what happened and plays back the tape of her speaking in an alien language. Beth insists that she's human and Gwen tries to reassure her. She wants to know if they can fix her but Jack warns that eventually she'll activate and send the gathered information back home to start the invasion. Beth wonders if they'll kill her and Gwen says that they've only killed aliens when it was the last resort. Beth insists that she wouldn't do anything but Jack insists that she'll have no choice.
The team meets and Tosh suggests they put Beth in cryogenic suspension and jam the implant. Jack warns that Beth would have to disappear entirely and her husband would never know. As they take Beth to the cryogenic chamber, she suddenly remembers what happened at her flat. She then has visions of nuclear devastation, and Jack realizes the alien implant is activating. As they prepare to freeze her, Beth asks them not to awaken her if they can't figure out a way to deactivate the implant. Gwen can't make the promise but Jack assures her he'll do it. Tosh prepares to short out the implant and the force field generator. Once they're out, they inject her with the freezing agent.
Businessman David is at home talking to his wife when he suddenly goes into a trance as his arm glows red and the implant is revealed. He starts to leave without a word, but when she persists in questioning him, he goes back and snaps her neck.
A paramedic is working on a patient when his right arm transforms. He walks away, abandoning his patient.
A housewife walking her baby transforms and abandons it in the street to be run over.
Owen and Ianto put the frozen Beth into the vault as Gwen looks on. As they leave, they're unaware that Beth's implant has activated, thawing her out. However, the power systems in the Hub go out and they quickly realize Beth has left through the tunnels. Jack wonders why Beth didn't shut down the entire facility given she knows everything about them. Owen realizes that the implant simulated her emotional and physiological responses, which is why the power flickered when she was upset. Jack insists they took off her off the alien network so Beth must have some other agenda.
beth goes to the hospital to see Mike and tell him that she loves him. She tells him that he has to go away and it's for his own good. Mike asks her not to go but she insists and tells him that she'll always love him. However, her implant activates, transforming into a bio-organic blade and stabbing Mike in the chest. Beth, still herself, calls for the doctor. Jack and Gwen arrive and get her away while the doctors rush in.
The altered David arrives at the Grainer household and kills the husband, Patrick, while his family looks on in horror.
The paramedic parks a petrol tanker outside the hospital, removes a disk from the arm implant, and attaches it to the tanker. Jack and Gwen are getting Beth out of the hospital when the tanker explodes. Tosh confirms it's disabled an emergency fuel pipeline. Owen confirms that Patrick Grainer was the city coordinator and they realize the alien takeover has begun.
The girl goes to a switch station and destroys it.
Jack and Gwen ask Beth how they can stop it. Beth explains she's cut off from the cell and doesn't know what the plan is. Gwen asks her to use the implant the way she did at Torchwood and Beth reluctantly does so. She manages to locate the last sleeper agent, the altered David.
Jack and Gwen follow Beth's directions and call the others using CB radio since the telephone line is out. They determine David is heading for an abandoned farm and Tosh tries to figure out what is there. She discovers that it's a coal mine that the Army sealed off in the 40s after placing ten nuclear warheads in it.
The military open fire on David when he arrives but his force field makes him invulnerable.
Jack, Gwen, and Beth arrive and Jack rams the sleeper agent. He gets out and tries to interrogate it but it stabs him through the chest. It tells Jack that they've learned everything about Jack and Torchwood. Gwen manages to shut down the implant transmitter and the force field and Jack shoots him dead. Jack asks when the others are coming but David says they're already here. He activates an explosive device and they get away just in time.
Later, the team plans to put Beth back into cryogenic suspension. Beth warns that she can feel the implant, pushing her out. She wonders what they'll do if she can't resist and Gwen assures her that they'll find a way. Beth says she's too dangerous and asks if Gwen has someone at home. Gwen admits she has a fiancé, and that she's hurt him. Beth tells her to imagine doing the same thing a billion times a day, and that she won't feel guilty once she transforms again. Gwen insists there's still hope but Beth transforms her arm into a blade and advances on Gwen, taking her hostage. Gwen tries to tell the others it’s a trick but the others believe they have no choice but to shoot her. Jack realizes what Beth did, and that she wanted to force their hand to make it easier for them.
Later, Jack contemplates Beth's arm blade and Gwen wonders if they've stopped the aliens' plan. She points out that they have the element of surprise and know how to disable the alien technology. Jack asks about the wedding and tells her to go home. Once she's left, Jack considers the blade and looks pensively up toward the sky.