Gwen has made a dinner date with Rhys, but Jack advises her to keep her mobile on, as Toshiko is tracking an alien energy signature somewhere in Cardiff and the team may be called into action at any moment. Gwen is irritated both by the thought that her date might be interrupted, and by the fact that Owen stands rather too close when they take the invisible lift up out of the Hub. She is thus in a bad enough mood when she arrives at the Indian Summer restaurant to find Rhys chatting with a beautiful young woman who turns out to be his co-worker Lucy. It takes Gwen a moment to recognise Lucy, who has lost a great deal of weight since they last met -- and although Rhys claims that Lucy just needed cheering up as she's going through a rough patch with her boyfriend, Gwen notices with some unease that Lucy is quite obviously flirting with Rhys.
As Gwen feared, her mobile goes off, forcing her to leave the restaurant early to investigate a fight at a nightclub. Five young men have killed each other for no apparent reason, and one of them is clutching a weird device made of a lavender-coloured metal. The police are irritated, as always, when Torchwood swans in, confiscates potential evidence and swans right back out again. Having done all she can at the scene, Gwen returns to the Indian Summer to find Rhys and Lucy apparently holding hands. Lucy explains that Rhys was just comforting her while she unloaded her boyfriend troubles, and leaves, assuring Gwen that she's lucky to have Rhys. Rhys admits that he was allowing Lucy to flirt with him, but insists that Gwen is the only woman for him. Gwen accepts that she's probably overreacting, but then Rhys tells her that Lucy might need a place to stay for a while...
Back at the Hub, Toshiko tries to analyse the device from the nightclub, but is distracted by a weird, mournful whistling. She eventually spots a pattern inside the device, and when she overlays the results of several different scans, she discovers that the circuitry has been arranged in the shape of an alien but humanoid face. She invites Owen to see for himself, and theorises that the device may have been built by a species for whom artistic design was more important than functional efficiency. She also asks Owen to stop whistling, but he tells her that he thought she was the one who was whistling. Puzzled, they trace the sound to the cells, where they discover that it's actually coming from the captive Weevil.
The next morning, Rhys takes a look at himself in the mirror and concludes that he's really let himself go. Fearing that this might be why the spark has gone out of his relationship with Gwen, he considers starting a diet and exercise routine, but this sounds like too much hard work and he instead decides to check out the diet clinic that has obviously worked wonders for Lucy. At the Scotus Clinic, located in an upscale area of Cardiff, Rhys undergoes a battery of tests before meeting Dr Scotus, a tall, thin man with fine blond hair. Scotus takes careful note of Rhys' home and work addresses before giving Rhys a blister pack containing two pills, one marked "Start" and the other "Stop." Scotus tells Rhys that the pills contain esters derived from a new breed of orchid he found in the Zambesi; he shows Rhys a photograph of his expedition, and it's obvious that Scotus has lost a great deal of weight since the photo was taken. Rhys accepts the pills, but he doesn't take the Start pill immediately, as something about Scotus has unnerved him; possibly Scotus' concern that Lucy's address and contact information is up to date, or possibly the way his hair waves in the air even though there's no breeze...
Torchwood investigates the death of a young Weevil in the warehouse district, and determine that it was attacked by something that chewed off its face. Owen and Tosh realise that their captive somehow sensed this Weevil's death and was in mourning. Weevils begin to cluster about the warehouse, and the team take photographs of the body and retreat, leaving the Weevils to their grief. Back at the Hub, Owen examines the photographs and concludes that the attacker severed a major blood vessel and drank the Weevil's blood before eating its flesh. Jack orders the others to keep an eye on the situation in case the predator moves on to humans, but Owen reveals that the tooth marks in the Weevil's flesh appear to belong to an ordinary human...
Tosh finishes testing the device from the nightclub, and concludes that it's an emotional amplifier, transmitter and receiver; it appears that one of the young men, Craig Sutherland, found it in an electronic parts store. Gwen visits the police to ask her old colleague, PC Jimmy Mitchell, for permission to view the club's CCTV footage, but he refuses to do so unless she shows him what Torchwood took from the crime scene. She agrees to do so, and he allows her one hour in the station's AV room, where she views the footage and concludes that Craig and Rick were trying to use the emotional amplifier to pick up girls; however, three youths wandered into range while looking for a fight, which is exactly what they got when the device picked up their aggression and fed it back into Craig and Rick. Torchwood declares the case closed, but Gwen smuggles the device out to show it to Mitchell. He buys her claim that it's a new type of videogame platform that the boys were working on, and that Torchwood needs to examine it to ensure that it doesn't contain stolen software. Instead of taking the device straight back to Torchwood, however, Gwen decides to take it home and see if she can use it to help put the spark back in her relationship with Rhys.
Toshiko believes that the emotional amplifier resembles other artefacts she's seen in the Archives. Ianto is already in the archives when she arrives, apparently conducting an audit; he leads her directly to the artefacts that she's looking for, telling her that she should just ask him in future if she needs to find something. Already creeped out by the alien technology all around her, Toshiko doesn't question his helpfulness or the odd sounds of movement she thinks she hears coming from deep within the Archive; in any case, she's rather more taken aback to find that the artefacts in question were apparently checked into the archive in 1955 by Captain Jack Harkness.
Gwen cooks a romantic dinner for Rhys and places the emotional amplifier in the centre of the table; it does its work, and Rhys and Gwen are in the bedroom before they get to dessert. In his post-coital haze, Rhys pops into the bathroom and takes the "Start" pill, determined to lose weight for Gwen's sake; however, thoughts of the Clinic remind him about Lucy, and when he returns to the bedroom, he impulsively asks Gwen if it's all right for Lucy to stay with them for a few days. This is exactly the wrong moment to raise that subject. Fortunately, Gwen realises just in time why their argument is escalating so rapidly, and she bolts into the dining room and switches off the alien device before their anger can spiral out of control into violence. Shaken, she spends the night on the couch and returns the device to Torchwood, but Jack confronts her and leads her to Torchwood's deep-sea aquarium, which uses alien technology to preserve and display creatures that normally only exist in the depths of the ocean. She admits what she did and apologises, and he's assured that she now understands, more than just intellectually, how an innocent night out ended in the violent deaths of five young man -- and just how dangerous the things that fall through the Rift can be.
Rhys is equally horrified by what nearly happened -- and he's also hungry, so hungry that he nearly empties the fridge even after eating the cold dregs of dinner and dessert. Disturbed, he contacts Lucy to ask if she's noticed any side effects to her diet; she claims that she hasn't, but she's lost more weight since he saw her at the restaurant, and she polishes off an entire large Venetian pizza with extra toppings while talking to him. A group of thugs then leap out of a white van and try to pull her off the street, but Rhys overcomes his surprise and drives them off. He tries to report the incident, but even Gwen's old partner, PC Andy, loses interest when he learns that Lucy's boyfriend is a heroin addict. Rhys thus calls Gwen for help. Both are relieved to learn that their partner doesn't blame them for the fight, and Gwen, knowing that it was really her fault, concedes that Lucy is obviously in trouble and agrees to let her stay at the flat while she sorts herself out. Lucy seems to be in a state of shock, but she insists that her boyfriend is too spaced out to arrange something like this -- or to care if his dealers were trying to get to him through her.
Toshiko has developed a way to trace Weevils through Cardiff based on their lower-than-human body heat, but rather than pick them off one by one, Jack is trying to get a feel for their migration patterns so he can deal with them all at once. When the system indicates that a large number of Weevils seem to be running scared, the Torchwood team heads for the docks to investigate. A pack of Weevils run past the team, but dismiss them as a threat; they're running away from something else, which leaps upon Toshiko and tries to bite off her face. It takes Jack, Owen and Gwen all working together to restrain Toshiko's attacker... who turns out to be a frantic, very thin girl named Marianne Till, who's so desperately hungry that she's already eaten a pigeon and a dog and nearly chewed the face off a man she picked up at a pub. Owen sedates her with carfentanyl, a tranquiliser designed for large animals that barely keeps Marianne in check. Torchwood takes her back to the Hub for study, and while leaving the cell area, Tosh notices that the captive Weevil is pressed up against the wall in its own cell, as far away from Marianne as it can get.
Gwen returns home to find Lucy sleeping on the couch. The next day, she returns to work, where Toshiko reports that Marianne seems to be a perfectly healthy young human in her late teens or early 20s -- albeit a perfectly healthy young human who's already eaten several large delivery pizzas from Jubilee and is still hungry. Ianto has told Marianne that she was given a date-rape drug in a bar, which is why she feels dazed and has been having weird dreams about eating people. Marianne's parents have filed a missing-persons report, and when Gwen refuses to back down, Jack orders Toshiko to fake up evidence that Marianne impulsively took a trip to Ibiza last night.
At Gwen's flat, Rhys is disturbed when Lucy begins flirting openly with him in his home. They're both ravenously hungry, but they've run out of food in the flat. Rhys takes Lucy out shopping, but her hunger gets the better of her, and when they return to the flat, she attacks him and takes a bite out of his cheek. Gwen returns home to find Rhys dazed and bleeding; he managed to push Lucy away, and she struck her head on the sofa arm and knocked herself out. While Gwen tends to Rhys' injury, Lucy revives and flees. Dazed, Rhys admits that he was allowing Lucy to flirt with him, but he never would have followed through on it, as he loves Gwen too much. Gwen takes him to hospital, where he is examined, treated, and released; in the process, she notices that he's lost weight, and he explains that he's taken the diet pills that Lucy recommended.
Unwilling to return to the flat while Lucy is still on the loose, Rhys rents a hotel room while Gwen visits Lucy's home in Grangetown. She arrives to find that Lucy has killed and eaten her boyfriend -- and is still hungry. Lucy attacks Gwen, who knocks her out, ties her up and contacts Torchwood. While waiting for Ianto to arrive, Gwen searches Lucy's flat; she finds a flyer for the Scotus Clinic and the blister pack, which still contains the Stop pill. After taking Lucy to the Hub, Gwen takes Rhys back home, but finds the blister pack with a missing Start pill in her bathroom. Worried, she returns to work, and Rhys, left on his own, eats an entire tub of margarine and discovers that the scar on his cheek is noticeably smaller.
Back at the Hub, Owen visits Marianne in the cells to scan her with the Bekaran deep-tissue scanner. Marianne seems to have calmed down somewhat after eating several large pizzas, including their cardboard boxes, and Owen returns to talk with her while Tosh analyses the results of the scan. He tells her that the man who drugged her may have infected her with a rare tropical disease called tapanuli fever, but Marianne is bright enough to notice some logical holes in his story. Despite himself, Owen feels a connection to the intelligent and frightened young woman, and he tries to keep her calm by telling her about himself and his fear of pointless, random death. Marianne thanks Owen for being the only person in this "hospital" to treat her like a human being, but then admits that she's starting to feel hungry again. Before Owen realises what's happening, she's chewed the flesh off her own fingers. Owen tries to get in and stop her, but she is unable to stop herself from attacking him; fortunately, opening up the cells set off an automatic alarm, and Jack and Tosh arrive in time to save Owen by knocking her unconscious with a fire extinguisher.
The next morning, Owen reports that Marianne has lost about half a stone since she's been locked up, and that he's had to amputate her fingers and chain her to the wall of her cell. Gwen reports her findings, and she and Jack visit the Scotus Clinic to see if there's a connection. Jack bribes the security guard to let them in, but the clinic is now deserted -- apart from a large three-headed black worm with blue rings around its body, which Jack is forced to shoot when it attacks Gwen and nearly throttles her. In another room, they find the body of Dr Scotus' receptionist, who apparently died when the worm forced itself out of her guts through her mouth. They also find a blister pack with both pills intact, and a list of Scotus' clients, which includes Lucy, Marianne, and Rhys.
Back at the Hub, Tosh continues to analyse the alien devices from the Archive and finds more images in the circuitry. More importantly, the computers finish analysing the results from the Bekaran scanner and determine that there's something living in Marianne's intestinal tract. When Gwen and Jack arrive with the blister pack, Owen cuts open the Start pill and confirms everyone's suspicions; it's not a pill, but the egg of a voracious alien tapeworm. The worm sucks all of the nutrients out of its host's diet, and can induce psychotic behaviour when its host begins to starve to death, driving it to keep eating whatever the cost. Jack decides to name the four worms they know about after the Beatles, for easy reference, but Owen, less amused, grabs one of the Stop pills and rushes to the cells to save Marianne. Unfortunately, she's managed to twist herself around and chew off her own arm, and has died of blood loss. Owen manages to hold himself together long enough to conduct an autopsy and remove Marianne's tapeworm, "Paul." The others store Paul in a tank and drip-feed it nutrients so they can study it, and Owen leaves them to it and walks out of the Hub, intending to get blind drunk even though he knows it won't help.
Jack decides that they have enough information and authorises Gwen to return Rhys' Stop pill. She tells Rhys that the tablets have been recalled after triggering psychotic episodes, and when he takes the Stop pill, the tapeworm immediately and messily flushes out of his system. Meanwhile, Tosh and Jack drive around Cardiff with the emotional amplifier until Tosh experiences an overwhelming sense of hunger near the Bute East Docks. They return to the Hub to find that, in their absence, Paul has metamorphosed into a flying insect. Owen has returned, and he and Ianto manage to contain Paul using another of the devices from Tosh's collection, a remote-taser that projects electrical shocks at a distance. Owen theorises that, when the worm's host dies, the parasite consumes its flesh and metamorphoses into a flying dart, which embeds itself in the nearest passing animal, killing it and planting its eggs in the body. The scavengers that consume the body also eat the eggs, and thus the creature's life cycle continues. Jack authorises Owen to give the remaining Stop pill to Lucy, and the team then prepares to visit the East Bute Docks and discuss matters with Dr Scotus.
Scotus has moved his operations to a former meat-packing warehouse. While Ianto waits outside in the SUV, the others split up in teams of two. Owen and Tosh find a room containing several dozen sedated patients with scars on their abdomens, and are confronted by two of the thugs whom Rhys earlier prevented from kidnapping Lucy. Owen and Tosh overpower the thugs and set off to tell Jack what they've found, but on the way, Tosh notices that one of the abandoned warehouse's cold-storage freezers is still operating. The freezer is full of dormant flying dart bugs, which presumably metamorphosed from the tapeworms that Scotus removed from the patients in the other room. Tosh and Owen realise that Scotus performed the operation rather than giving his patients Stop pills because he intends to use the sedated patients as hosts for more eggs. The warmth from the hallway then revives the insects, and the automatic door's safety cut-outs prevent Tosh from slamming it shut while the bugs are halfway through. She and Owen are forced to retreat as the bugs spill out into the corridor.
Gwen and Jack search for Dr Scotus, while carrying "Paul" in a birdcage with a tarpaulin over it. They find him in one of the warehouse's offices, but are captured by another of Scotus' thugs and forced to drop their weapons. Nevertheless, Scotus seems willing to explain himself. He used to be a veterinarian, who found the first tapeworm -- which presumably came through the Rift -- while examining a cow that had gone mad and started eating other cows. Scotus took the worm home to examine it, but it metamorphosed, killed his dog and planted its eggs in the body. Scotus began to experiment with the eggs, and eventually worked out the parasite's life cycle and how to turn it to his advantage. He believes that he's found a way to control the creatures' growth using specific herbs, but there are still a few kinks to work out, which is why he had to abandon his other clinic; his receptionist forgot to take one of her control pills, and her parasite became agitated and burrowed its way out of his body.
Scotus admits that he's hired criminals to kidnap his clients, and Gwen realises that he intends to use them as hosts for more eggs. Scotus insists that early medical testing always results in death and complications until the drugs' effects are fully understood, but Gwen sees his fine blond hair waving in a breeze that isn't there and realises that the hairs are tendrils from Scotus' own parasite. It's become a part of his body's nervous system, and his arguments are just conscious rationalisations of unconscious impulses driven by the parasite's needs. Jack has heard enough, and before the goon guarding Gwen can react, Jack and Gwen trigger both the emotional amplifier and the remote-taser that they've planted in Paul's cage. Paul's pain is amplified and transmitted to everyone within range, incapacitating Scotus and the goon long enough for Jack to pull himself together and restrain them. Owen and Tosh then arrive and warn Jack about the other insects in the corridor, and Jack, theorising that the insects hunt by body heat, orders Owen to blast him with the contents of the nearest fire extinguisher. The blast lowers Jack's body temperature, and he walks into the midst of the swarm and picks the insects off, one by one, thus saving the patients in the other room.
Owen tends to Scotus' victims, telling them that someone drugged them in a bar and stole their kidneys. Scotus is unable to bring himself to take a Stop pill, but agrees to let Torchwood inject with a pill dissolved in a solution; however, he is too deeply entwined with his parasite, and does not survive its death. Lucy has survived, but is psychologically scarred and may never fully recover. Gwen's contacts in the police force raid the warehouse and arrest Scotus' goons, and Gwen takes some time off and goes on holiday with Rhys. Owen hasn't dealt with Marianne's death, but Jack feels sure that Owen will pull through, as he always does. Meanwhile, Tosh finishes her work on the alien devices and concludes that the images in the circuitry, when arranged properly, tell the life story of the designer. The last in the sequence shows the creature standing next to its own offspring; whereas humans do all they can to prevent the slow decay of their own bodies, this alien, at least, knows that it's through our children that we survive.