Jack calls Owen in to work early, but as he leaves the house, a milkman named John Davies says hello, introduces himself and starts talking about his wife and kids. As Owen pushes past the babbling man to get to his car, a postman stops and says hello to him. Outside the Hub, Gwen encounters an early-bird tourist who says hello to her, introduces himself as Kurt Stoltzman from Hamburg, and tells her his mother's maiden name. Owen and Gwen report the strange encounters to Jack, but for the moment he's more concerned with the energy readings Tosh has detected. Last night, something came through the Rift and blanketed Cardiff with a powerful energy field, but they can't pinpoint the source and have no idea what the field's purpose is.
A woman named Gemma Sands is held up on her morning commute when her newsagent and an old man begin holding a protracted conversation about each other. A slightly creepy guy whom Gemma has noticed leering at her at the bus stop seems to be trying to make eye contact with her, so Gemma pushes up to the front of the queue to pay for her newspaper and get out of the shop. The vendor and the old man respond by saying hello, introducing themselves as William McGarrish and Vernon Hines, and asking her about herself. Gemma heads out to the bus stop, where a queue is forming behind a woman who seems to be having a conversation with the driver. Gemma boards her own bus, but as she sits down and starts to work on her sudoku, the creepy man from the queue sits down next to her and says hello.
Elsewhere, a schoolboy named Ishram Kapoor is beaten up by white thugs when he walks up to them, says hello and starts talking about himself. 26-year-old supply teacher Alan Kennedy walks into his classroom, only to have all of the children stand up and say hello to him in unison. Outside the train station, a woman approaches cab driver Vic Royce, says hello, introduces herself as Jane Broading and starts telling him all about herself. The usually short-tempered Vic thanks her, gets into his cab and drives off alone. As he drives, he passes a bus that's pulled over to the side of the road; paramedics are working on a slumped figure inside while police are questioning the traumatised Gemma Sands, who panicked and stabbed the strange man in the neck with her pen when he wouldn't stop talking to her.
The Torchwood operatives determine that the energy field is registering on the PK scale. Gwen and Jack both score five out of five on an impromptu Zener card test, which implies that the field is linking their minds together. Ianto reports that telephone usage has gone up 87%, and most of the calls seem to consist of people saying hello and talking about themselves, their friends and their families. Jack theorises that whatever came through the Rift has made first contact and is using the telesensual field to make people talk about themselves so it can learn about the planet it's landed on. But what will it do with that information? And what's going to happen to the city if people keep talking about themselves instead of doing their jobs?
At the hospital, Dr Cornfield stops in the middle of an appendectomy, says hello to his patient, and walks out of the operating theatre. Staff nurse Angie Mercer is horrified, but anaesthetist Bob Gordon and doctor's assistant Benjamin Okeno respond by saying hello to her. When she calls for help, the person on the other end of the phone says hello and introduces herself as Lucy Philpott. Outside the hospital, an ambulance has stopped some distance from the door while the driver chats with an old man in a wheelchair. Elsewhere in the city, a man named Brian Hatter is involved in a car accident, and the other driver steps out of his car, says hello, and collapses with blood streaming down his face. Brian asks a bystander to call the emergency services, but the bystander says hello and introduces himself as Terry Spicer. Before Brian can use his own mobile to call for help, his wife Julie phones him up, says hello, and starts telling him all about herself.
Vic Royce arrives at a disused garage, where several other people have gathered in front of a pulsing light: Alan Kennedy, Janie Clowes, milkman John Davies, policeman Greg Randall, and Tesco's cashier Linda Sosa. Since their minds are particularly receptive, the light has appointed them its heralds. But as it feeds information to them, they sense that something is wrong. Elsewhere, the inexperienced young PC Tony Pratt is left helpless and alone when his partner Clive Bevan steps out of a moving car to join a mob of people saying hello to each other. At the shopping mall, security guard Ivor Brown lashes out in panic at the mob of people closing in on him and saying hello, and in the confused violence of the moment, he falls down the escalator, taking a number of people with him.
Jack, Owen, and Gwen don silver bracelets to protect themselves from the telesensual signal, and head out to locate and shut down the source while Tosh and Ianto guide them from the Hub. Outside, Cardiff city centre looks like a war zone, with people's belongings lying abandoned in the street and shop fronts smashed by opportunistic looters. Columns of smoke are rising over the city, from unattended ovens and ironing. Jack, Owen, and Gwen run into a panic-stricken man who doesn't understand what's going on, and when Jack inadvisably says hello and introduces himself, the man hits him with a brick and runs away screaming. Nevertheless, Jack refuses to let Owen arm himself for protection, insisting that the people they're encountering are innocent victims of the telesensual field.
The operatives stop to get their bearings near an abandoned gas board van, and overhear a conversation between Sally Gatterson and John Purvis. Sally likes soup and has never had a proper orgasm; John watches the Playboy Channel while his wife is out, and he once struck a boy at a zebra crossing but kept driving. They are joined by Ronald Lucas, who hits his wife, once made a balsa model of the Cutty Sark, and has unprotected sex with prostitutes twice a month. The information people are giving about themselves is becoming more and more personal, and the Torchwood operatives worry what the alien contact will make of it all. Gwen then hears a baby crying and rushes off, with Owen in pursuit, to find that it's been abandoned in its carriage. Jack remains behind to comfort a crying teenaged girl who appears unaffected by the field -- and when the unattended gas main next to the abandoned van explodes, he and the girl are engulfed in the fireball. Sally, John, Ronald, and several other greeters are killed in the blast, but Jack's body shields the girl, and when he revives he tells her that he just got lucky. They very carefully introduce themselves to each other and leave it at that; the girl's name is Lori Coleman, and she agrees to lock herself in the nearby department store's changing rooms with the baby and look after it until Jack and his team sort things out. Elsewhere, PC Tony Pratt has been driving aimlessly about, watching helplessly as Cardiff descends into chaos. He encounters a group of teens joyriding a 4x4, but lets them go, conceding that it probably is the end of the world and that he'd do the same if he were them.
Tosh and Ianto narrow down the field's source to the northern suburbs, but before they can pinpoint its exact location, the source strikes back at them, nearly burning out the Hub's computer system before they can shut it down. The light has interpreted Torchwood's analysis of the field as an attack, and since the heralds can see fires raging and violence erupting in the city, the light concludes that this is a proven hostile contact. The heralds thus arm themselves and head out into the city to locate the enemies of the light. Meanwhile, Tosh directs Owen to Pentwyn, and Jack, whose earpiece was destroyed by the explosion, picks up an abandoned mobile so he can keep in touch with the Hub. Unfortunately, his PDA has also been destroyed, leaving him with no way to narrow down the source of the field -- and even worse, his protective cuff has been damaged, which means he is now vulnerable to the telesensual field. A police car turns the corner and approaches the team, but Jack orders Gwen and Owen to start talking to him. Bemused, they allow the police car to pass by without asking for help. Once it's out of sight, Jack explains that he saw a milkman driving the car while the policeman himself rode shotgun. However, he doesn't reveal the real reason he was wary: he sensed that they were dangerous because the field is starting to affect him. Tony Pratt then drives past Jack's team, recognises Gwen and stops, desperately asking her for an explanation. Realising that Tony is still unaffected by the field, Jack asks him for a ride to Pentwyn.
A man named Lionel Mellie flags down the other police car, begging for help. But as Jack had sensed, the car is being driven by heralds: John Davies and Greg Randall. Greg asks Lionel if he's seen the light, and when Lionel doesn't understand the question, Greg bashes Lionel's skull in with his telescopic baton. Elsewhere, the joyriding teens who encountered Tony Pratt break into an unattended off-licence for the fun of it, but Vic Royce finds them and murders the three boys -- Barry, Lol, and Pat -- with a length of pipe. The girl, Colleen, flees screaming, but runs into Linda Sosa, who hits her in the head with a spanner.
Jack allows Tony to believe that the chaos is the result of a radiation leak that's affecting people's minds; however, Tony's relieved babbling is getting on his nerves, and Gwen and Owen notice that he's wound tighter than usual. Greg Randall and John Davies then encounter them, and although Tony at first assumes the other policeman is here to help, he follows Jack's instructions and speeds off when Greg starts smashing in his car's windows. John Davies puts his foot down and chases Tony through the streets, trying to run him off the road. Tony is forced into the shopping precinct, and swerves at the last minute to avoid a row of concrete bollards -- but John Davies, right behind him, drives straight into them at 83 mph. Greg Randall sees the fatal crash and calls on the other heralds for assistance, and a driver named Tommy Kincannon arrives with his articulated lorry just as Tony's car, damaged in the chase, sputters to a stop. Tony, Gwen, Jack and Owen manage to get out of the car before the lorry hits it at 60 mph, but Tony and Gwen are separated from Jack and Owen, who must run in the other direction when the lorry circles back around towards them.
Jack and Owen duck into an alley too narrow for the lorry to chase them through, but Tommy gets out and chases them on foot. Vic and Linda join the chase, and Jack tells Owen to keep running while he turns to face the heralds. They are suspicious when Jack says hello to them, but when they look into his mind, they see that he has genuinely stopped resisting the field's influence -- and unlike anyone else they've encountered so far today, he knows about the Rift. While Tommy and Vic keep hunting for the other enemies, Linda takes Jack back to the garage to meet the light. It feeds information into his mind as it has done with the other heralds, and he thus learns that this is indeed a first-contact protocol gone wrong. And when he sees the source of the light, an alien sphere the size of a small car, he discovers why.
Gwen tries to keep Tony from panicking while using her PDA to trace the source of the field, but fails at both when Greg Randall catches up to them. She and Tony duck into a nearby school, where the students and teachers are all saying hello to each other. When Greg arrives, he uses his authority as a herald to get specific information from the greeters, who point him towards the computer room where Tony and Gwen are hiding. Despite his fear, Tony distracts Greg, enabling Gwen to surprise him and knock him unconscious. Gwen chains him to the radiator with his own handcuffs and takes his telescopic baton, but then Vic Royce catches up to them. Gwen smashes open the school window with the baton, and she and Tony make a run for it -- but Tony runs right into Tommy Kincannon, who knocks him unconscious. Gwen is forced to stand and fight both Tommy and Vic to save Tony's life, but Vic breaks her hand with a blow from his pipe and moves in for the kill.
Tosh and Ianto have been repairing the Hub's computer systems so they can try communicating with the source of the field. Owen calls back to the Hub to tell them what's happened, but is then confronted by Janie and Linda. Before they can kill him, Jack arrives and tells them to leave it to him. Once he's alone with Owen, however, Jack reveals that he's still in control of himself; the field has calibrated itself to the brainwaves of 21st-century humans, but Jack's are somewhat different. He explains that, as he'd suspected, the field is an information-gathering system, intended as part of a non-hostile first-contact protocol -- but the information it's gathering isn't going anywhere, because the ship's pilot was killed when his craft crashed through the Rift. The ship's computer is aware that something is wrong but isn't sure what, and it has therefore classified this planet as a hostile contact and is acting to protect itself and its owner. Owen passes on Ianto and Tosh's suggestion that they try communicating with the field, and Jack calls up Tosh and tells her to broadcast a stream of random gibberish to his mobile. He takes the mobile back to the garage and holds it up to the alien craft, and when it receives the gibberish, it shuts down.
Back in the Hub, Ianto and Tosh confirm that the telesensual field has collapsed. Outside the school, Vic comes to his senses before he can kill Gwen, and he and Tommy stare about themselves in confusion, unsure where they are or how they got here. Gwen assures the recovering PC Tony that the nightmare seems to be over. Back at the garage, Jack explains to Owen that the alien ship was unable to interpret the random data that it was receiving, and thus concluded that it had contacted something too alien for it to understand. Since it was unable to fulfil its contact protocols and communicate with the alien signal, it shut down to await further instructions. As the confused heralds wander away from the garage and Jack and Owen prepare to take the alien ship back to secure storage in the Vaults, it begins to rain, and Jack and Owen hear the sound of people across the city saying goodbye.