Gwen and Rhys go bowling and afterward, Gwen tries to explain what her new job is without telling him what it really is. They're interrupted when a flaming object passes overhead in the sky. Gwen grabs Rhys and runs after it and they watch as it crashed down outside of town. Gwen gets a page from Torchwood and tells Rhys she has to go to work.
The team heads for the crash site but discover that the military are already there. Gwen introduces herself as a member of Torchwood without success, but Jack arrives and makes sure the military accepts their authority. They go to the meteorite and check the item. Owen believes it's standard space debris and they start taking standard samples. Gwen watches without knowing what to do and Owen starts mocking her. She snaps and throws him a chisel, but it hits the side of the meteorite. A purple gas leaks out and drifts up into the sky.
At an alley outside a nightclub in town, a woman, Carys, tries to call her boyfriend, who has dumped her. The purple gas closes in on her as she tries to retreat. It enters her body. A minute later, Carys walks up to the bouncers, demands readmission, and kisses one of them to convince him to let her in. Inside, Carys catches the eye of a boy and a minute later they're in the women's restroom. They start kissing and have sex. As soon as the boy reaches completion, he bursts into energy and Carys absorbs the particles into her body.
At the Hub, Gwen apologizes and insists she'll deal with it. Jack says they all make mistakes and she needs to get over it. Ianto gets a report on the nightclub death and they go to investigate. One of the PCs, Andy, recognizes Gwen and admits that they've all been wondering what happened to her. As she goes inside, she asks him to say hi to everyone.
In the restroom, the bouncer reveals that he was watching on the security cameras and saw the entire thing happen. They get the recording from him and watch it, but the man doesn't know anything else about who Carys is. Jack starts the cover up and tells Ianto to get a dead body out of storage. They check the alleyway and find the same trace elements in the alleyway that they found at the crash site. Jack spots a surveillance camera and they play it back and watch how the gas took over Carys. Gwen blames herself for the boy dying since she unleashed the alien.
Carys is at home having breakfast with her father Ivan. She admits she can't remember what she did the previous night.
At the Hub, Jack tries to track the incoming meteor trajectory while Gwen realizes that none of them have partners. She wonders how they turn it all off.
At home, Carys cries in the shower.
At the Hub, Tosh runs the CCTV image from the camera and comes up with a 119 possible matches to Carys. There's no match for the fingerprints they found and Tosh starts backtracking all the CCTV cameras to see where she started the evening.
Carys is dressing when she writhes in agony. A deliveryman arrives at the door and she goes to greet him. Carys pulls him inside and starts having sex with him. However, the Torchwood team burst in, order the deliveryman out, and try to take her captive. Carys makes a run for it but Owen manages to capture her in an alien inflatable cell. They secure Carys and Jack makes sure to recover the technology that Owen took without permission.
Back at the base, Gwen places Carys in a cell. The girl has no idea where she is or what she's done. As Gwen starts to explain, Carys writhes in pain again and then moves to the edge of the cell and recognizes Gwen as the person who broke her ship. The alien says it's not interested in enslaving humanity. All it wants is the energy of climax that lives off of. Carys starts to writhe again, asking for help, and Gwen unlocks the cell door and goes in. Carys begs her for help, but then the alien takes over, slams her into the wall, and starts kissing her.
In the Hub, Owen sees what's happening on the monitor and settles back to watch. Meanwhile, Gwen grabs Carys and starts kissing back. Jack finally realizes they should get her out of there, while Owen starts recording. However, the alien insists that it has to mate with a man and shoves Gwen away. Carys tries to fight for control while Gwen gets a call and backs out of the cell. Rhys is there and wants to know when she'll be home and how the job is going. Jack and Tosh arrive and Gwen lets them know she's okay. After Gwen hangs up, she gives one last look at Carys and leaves.
In the Hub, Owen applauds Gwen, who grabs him and insists Carys is an innocent victim. Owen wonders why she's suddenly so concerned. Ianto arrives with Chinese and they relax together. Once Jack steps out to pee, the others want to know what Jack has told Gwen about his sexuality and origins. They all have their own guesses. Gwen hears a sobbing noise and they check the monitor where Carys is crying in her cell. When Gwen wonders they're doing while Carys is suffering, jack asks her to tell them what they're missing. Gwen brings up the records on Carys' life and says they have to free the girl. Jack is impressed, until Gwen suggests they bring in Ivan to connect with Carys and help her fight back. Jack isn't thrilled with the idea of endangering a civilian. Tosh reports and tells them that the alien is secreting airborne pheromones. They realize any male is vulnerable, and that Owen is at large. They run down to discover that Carys has escaped and Owen is naked, handcuffed, and missing his security card.
In the Hub, Jack locks down the facility and cuts off Carys' escape. She grabs a severed hand in a jar and she threatens to destroy it. Backing away, she gets to the upstairs entrance. Jack demands that she turn over the hand and Carys throws it on the floor. Jack picks it up while Carys runs outside. Gwen and Tosh go after her but lose her in the streets.
Later, Jack gets the hand back in safety. He refuses to discuss why it's so important and tells Gwen to use her contacts in the police to find Carys. Owen calls them in to tell them that the bioscan reveals that gasses will eventually cause Carys to explode. They try to figure out what she would do next but aren't sure if Carys or the alien has control. Tosh figures that the reproduction instinct will be dominant and she knows where the alien would go.
Carys goes to see her ex-boyfriend Eddie Gwynne, her first. He insists that he won't leave his current girlfriend, Bethan, and Carys grabs him and starts having sex.
Jack and Gwen arrive at Eddie's apartment and discover he's been reduced to dust. The team looks to Gwen for answers and she remembers that Carys works as a receptionist… at a fertility clinic.
Carys arrives at the clinic and tells the receptionist that the alien needs more. She knocks out the receptionist and then starts working her way through the sperm donors.
As the Torchwood team approaches the clinic, they give Gwen a gun. They figure that if they can isolate it from Carys' body, the alien will die in Earth's atmosphere. They go inside the clinic and get the remaining men out. Owen and the others manage to corner Carys, still struggling for control. She says she only needs one more victim. Jack decides to give up his own surplus of energy to save her and kisses her. Carys' body glows with energy and Gwen volunteers to let the alien use her body until they can find a cure. Jack agrees to let Gwen make up for her mistake. The alien gas emerges from Carys' body and approaches Gwen. She drops the alien confinement unit on the floor. It activates, containing the alien until it dies. Gwen gives Jack a thank-you kiss, leaving him momentarily taken aback.
Later, Gwen and Jack take Carys home and reunite her with her father.
Back at the Hub, Gwen takes down her pictures of Carys and Jack wonders why she's still there. In response, Gwen asks who he really is, and how he can't die and knows so much. Jack tells her to just go home and be normal, and provide them with the human perspective they need.
At home, Gwen eats lasagna with Rhys and makes love.