Joe and Jackie Hargreaves are returning home to Cowbridge after visiting friends in Llandaff. However, they run into a bank of fog on the A48 and find themselves suddenly transported several miles back to the outskirts of Cardiff. As they approach the fog bank for the second time, a man suddenly looms up out of the fog, and despite Joe's efforts to avoid the figure he hits it and skids off the road. Jackie is injured in the crash, and Joe, unable to get a signal on the mobile, tells her to wait while he checks on the man he hit. Moments later, Jackie hears Joe screaming in terror and agony, and then she too is attacked -- by zombies.
A party of drunken Cardiff University students decide to take a joyride in a yacht belonging to their ringleader Steffan's uncle. Curtis, Stan, and Greg are happy to go along with whatever Steffan wants, and Curtis's roommate Toby reluctantly gets talked into joining them, although privately he's decided it was a mistake to hang out with this rowdy bunch. The students get into Penarth Marina and take the yacht out into the Bristol Channel, but run into a fog bank and suddenly find themselves back in sight of Cardiff. As they try to figure out what's happened, an army of zombies swarms up out of the sea and attacks them.
Gwen and Rhys are woken by the sound of a scream from outside their flat, and investigate to find that a tramp is eating their neighbour's cat -- or so they think until he looks up and is revealed to be a walking corpse. Bullets have no effect on the dead man, and Gwen and Rhys retreat to the flat to call Jack. Meanwhile, waitresses Sophie Gould and Kirsty Lane finish their shift at the El Puerto restaurant, and as they head out to hit the clubs, they see the Torchwood SUV join a cluster of police vehicles in Penarth Marina. The yacht has drifted back to shore, and upon seeing the condition of the students' bodies, DI Myers has called in Torchwood to investigate. DS Kathy Swanson isn't pleased, but allows SOCO Guy Baker to show the crime scene to Jack and Ianto. The evidence suggests that that students were torn apart and eaten alive by other human beings, and Jack compares this to Gwen's report and concludes that, ridiculous as it sounds, Cardiff is being attacked by zombies.
PC Andy Davidson and his new partner, PC Dawn Stratton, respond to a report of a disturbance at a party on Gabalfa Road. Apparently, a man burst into the party and bit a girl named Hayley, and it took six men to bundle the attacker into the cellar. Andy and Dawn try to arrest the gatecrasher, but he slips out of Andy's grasp and bites Dawn before they can overpower and handcuff him. They realise that there is something unusual about him, but don't work out what it is until they emerge from the house to see an army of zombies attacking the party guests and paramedics. Unable to rescue the zombies' victims, Andy and Dawn retreat to their car and call for backup, and Andy, realising that this is a situation for Torchwood, also contacts Gwen.
Trystan Thomas and his wife Sarah are expecting their first child, and tonight is the night that Sarah's waters break. They contact their midwife, Rianne Kilkenney, who was about to go home but instead remains at the hospital. While waiting, she strikes up a conversation with student Nina Rogers, who injured her foot while drunk and has come to see if it needs stitches. As they chat, people begin to run in from the car park, claiming that a crowd of strange-looking people is approaching the hospital. Nina and Rianne go up to the maternity ward for a better view, and see that the hospital is surrounded by zombies; as they watch in horror, a man arrives at the hospital in his car, and the zombies tear him apart and eat him alive. Unaware of the situation, Trys and Sarah Thomas are approaching the hospital when they see a group of people crouched over a body in the street. Assuming that this is an accident scene, Trys stops to help, and is attacked by zombies. Desperate, he tries to get back to his car, but only succeeds in drawing the zombies' attention to his wife.
Back in the Hub, Ianto notices that an alien orb that the operatives recently acquired is behaving oddly; it has been regenerating slowly over the past few weeks, but the speed of repair seems to have increased. He puts this mystery aside for the moment, however, since the zombie attack seems to be a much higher priority. He and Jack scan the city for Rift activity, and discover that Cardiff has been sealed off behind a time barrier. Zombies are appearing all over the city, but they do not seem to be coming through the Rift. In order to find out what's going on, Jack and Ianto must capture a zombie for analysis.
Gwen and Rhys try to drive to the Hub, but see a police car swerve and crash while trying to avoid what appears to be a group of people in the middle of the road. Rhys also crashes while swerving to avoid a zombie, and the police in the crashed car are attacked and devoured before Gwen can save them. The zombies then turn on Gwen and Rhys, but Gwen shoots the approaching zombies in the head while Rhys defends himself with a golf club from the car boot. Gwen calls Jack to report her situation, and decides to change her plans and go to police HQ to help them maintain order in the besieged city. On their way, Gwen and Rhys see a group of zombies attacking a café near Callaghan Square and try to help the people inside; however, the rear entrance has been jammed shut by a terrified man who hopes to keep the zombies from noticing him by sacrificing the people in the café. Gwen and Rhys are unable to free the back doors in time, and hear the people in the café being killed. The furious Gwen restrains herself from shooting the man in cold blood, but leaves him to his fate.
Sophie and Kirsty are heading for the clubs when zombies attack their taxi, killing Kirsty and the taxi driver, Winston. The terrified Sophie manages to run back home, but while she's trying to dial 999, a zombie walks into the flat carrying the severed head of her roommate, Kim. Sophie leaps out of the window to safety, but falls badly and twists her ankle. Meanwhile, Andy drives the ailing Dawn to St Helen's Hospital for treatment, only to find it surrounded by zombies. He decides to take her to police HQ instead, and nearly runs over the fleeing Sophie on the way before he realises that she's a scared teenage girl and not another zombie. He gives Sophie a lift to the police station, only to find that it is also under siege. With nowhere else to go, Andy takes Sophie and Dawn back to his house, where he tries to give Dawn first aid while the shaken Sophie takes a hot bath. Dawn suddenly turns on Andy, her eyes glazed and feral; however, Sophie emerges from the bathroom just in time and wraps a towel around Dawn's head, restraining her until Andy can lock her in the bedroom. As Andy and Sophie recover, Andy hesitantly asks Sophie out on a date once this is over.
Jack and Ianto go out hunting zombies, and find the pack attacking Trys and Sarah Thomas just in time to save the couple's lives. Trys is badly injured and Sarah is going to give birth at any minute, so Jack and Ianto decide to take them back to the Hub. First, Ianto helps Jack to capture a female teenage zombie, but as they try to lock her in the boot, a zombie child darts out from beneath the SUV, and the surprised Jack falls back, hits his head on the asphalt and passes out. He does not die, which means that he has a headache when he awakens back at the Hub to find that Ianto has placed the injured Trys in the hothouse and is trying to help Sarah deliver her baby in the boardroom. Jack has had experience in this area and steps in to help, but afterwards, Ianto points out that this highlights their urgent need for a new medical officer. He also wonders what would have happened to Jack if the zombies had torn apart his body and eaten him; Jack tries not to think about this. He and Ianto study their captive zombie, which Jack names "Mildred," and discover that it is not a reanimated corpse but an artificial construct made out of ersatz matter and given shape by some other governing intelligence.
Trys emerges from the hothouse and attacks Jack and Ianto, apparently turning into a zombie himself. They overpower and restrain him, but while moving Mildred out of the way so they can run tests on Trys, they walk her past the alien pod that Ianto had noticed earlier -- and it reacts to her presence. Before dealing with this, Ianto and Jack examine Trys, and determine that he is not physically turning into a zombie but is instead reacting psychosomatically. Ianto then checks out the report on the mysterious pod, which demolished several buildings when it crashed in Splott a few weeks ago. He discovers that most of the fatalities were attendees at a late-night zombie movie marathon at the Regal Cinema on Railway Street. The only survivor, 22-year-old Oscar Phillips, is in a coma in St Helen's Hospital. Ianto and Jack set off to investigate, taking the pod with them, restraining Trys in the Vaults, and giving Sarah a mobile and their assurances that the Hub is the most secure installation in Cardiff.
Andy contacts Gwen to warn her that the police station is under siege, and she and Rhys decide to head for the Hub instead. Gwen tries to "borrow" a car to avoid the zombies, but the car's owner, Keith Samuels, spots her trying to break into it. The ensuing confrontation is interrupted by zombies, and Gwen and Rhys must retreat into the Samuels' house and barricade themselves in with Keith, his wife Naomi and their daughter Jasmine. Gwen and Rhys try to keep the family from panicking, but the zombies break into the house and they are all forced to retreat into the attic. The terrified Naomi lashes out at Gwen for bringing this into her home, but Gwen manages to calm her down. She calls Jack, who tells her that he and Ianto are going to St Helen's Hospital; Rhys then uses his own mobile to call a friend who owes him a favour.
Ianto and Jack arrive at St Helen's to find it surrounded by zombies. The zombies aren't trying to break in, but they grow agitated at Jack and Ianto's approach, presumably because of the presence of the alien pod. Jack insists upon getting the pod to Oscar Phillips somehow, and he and Ianto emerge from the SUV and begin shooting. They are heavily outnumbered and nearly overwhelmed, but at the last moment, a Coast Guard helicopter arrives on the scene and lowers a ladder to them. Once safely in the helicopter, they discover that it's being piloted by Rhys's mate Nobby, who has already rescued Rhys, Gwen, and the Samuels. Nobby lands on the roof of the hospital, and the zombies respond by storming the building; however, they ignore the screaming staff and patients, instead making their way to the upper floors of the hospital in pursuit of the alien pod.
Jack, Ianto, Gwen, and Rhys track down Oscar Phillips, who rises from his bed despite being in a coma. Jack gives Oscar the pod, and a wormlike creature emerges from Oscar's mouth and enters the pod, which begins to float in the air under its own power. Speaking through Oscar, the alien identifies itself as Leet of the Cellacoi, a peaceful space traveller who crashed to Earth by accident; it merged with Oscar's body to survive, and both Oscar and Leet have been keeping the other alive ever since the crash. Leet has the ability to create solid objects out of pure thought, and once its life pod had grown strong enough to call out, Leet created drones to track down the pod and return it to him. Unfortunately, the drones took the form of zombies from Oscar's memories of the movie marathon he'd been watching at the time of the accident and ran out of control, and Leet had to seal off Cardiff to limit the damage they were causing. The zombies break into the ward to take the pod, but Leet communicates with Oscar's sleeping mind, explains what's happened and wakes him from his coma. Realising that he is the one giving the zombies form, Oscar grabs Gwen's gun, shoots out the window of the hospital ward, and leaps through it to his death several storeys below. When he dies, every zombie in Cardiff vanishes into thin air.
Trys Thomas awakens with no memory of what happened after he was attacked; however, Ianto left a mobile in his cell in his cell in the Vaults, and when Trys calls for help, Ianto assures him that he has a healthy baby boy and that the Torchwood operatives will be back soon to release him. PC Dawn also awakens to find herself locked in Andy's bedroom, and Andy and Sophie must explain the situation to her. Nina Rogers and Rianne Kilkenney emerge from hiding as the Torchwood operatives leave, and Jack assures the two women that the nightmare is over. As he leaves the hospital, he spots a familiar face -- an elderly patient named Alexander Martin, whom Jack decides to get in touch with later. Five days after the zombie attack, Jack and Gwen watch Oscar's funeral from a distance, and Gwen leaves snowdrops on his grave, knowing that the deaths weren't his fault and that his sacrifice saved the city.