It's nearly Halloween, and Miss Bullivant, the sacristan at Holy Innocents church, believes that Father Ninian should use his next sermon to address the matter of children playing with icons of demons and monsters. But he won't get the chance, because a young man named Gareth is waiting in the confessional, and he hasn't forgiven Father Ninian for what he did when Gareth was an altar boy. Gareth flicks a playing card through the confessional's grille and departs as a monster appears in a blaze of light and attacks Father Ninian. Outside, Jack and Ianto are tracing an energy spike and trying to avoid the attentions of investigative reporter David Brigstocke, who believes that Torchwood was involved in the death of his friend Rhodri. When they see a Weevil run out of the church, Ianto gives chase while Jack enters the church. Inside, he finds that Father Ninian has been savaged to death -- and is clutching a MonstaQuest playing card with a picture of a Weevil on it. The Weevil gets away from Ianto, who returns to find that Brigstocke has followed Jack into the church; however, Jack and Ianto give him the slip while he's comforting the sobbing Miss Bullivant.
Gwen has ordered Rhys to stay away from Pendefig Mall while she visits it to try on her wedding dress, but he secretly visits with Banana Boat, his best man, to shop for wedding supplies. Rhys parts company with Banana Boat, promising to meet up with him at the Millennium Stadium later, and starts looking for a gag gift for his best man. While doing so, he glimpses a commotion caused by what seems to be a teenager wearing a Halloween mask. Thinking no more of it, he continues on to the games shop, where a party of college students are playing a MonstaQuest tournament. The game's "Toothsome" monster resembles Gwen's description of a Weevil, and Rhys, amused, decides to buy a pack of the cards for Banana. As he does so, the mall's fire alarms go off, and everyone in the store is forced to evacuate.
Elsewhere in the mall, Gwen is forced to ditch her friend Megan and give chase when she sees a Weevil running through a crowd of shoppers. Young Jenny Bolton, seeing the creature and assuming that it's a teenager wearing a Halloween mask, photographs it with her mobile phone -- and it vanishes. Two more Weevils then tear through the crowd, slashing at the shoppers with their talons. Jenny is forced through two fire doors while fleeing from the panicked stampede, and in the mall's service corridors, she is attacked by a giant bat-like creature that has already killed one of the Weevils. Jenny inadvertently presses her mobile's speed-dial as she dies, calling her mother Shona, who is on a bus heading to the mall. When Shona takes the call, the Weevil that Jenny had photographed materialises in a blaze of light and slashes its way up to the front of the bus, where it kills the driver. The bus swerves out of control and crashes into a construction site near the Withington Hotel and Wendleby's department store.
Gwen arrives in the service corridor just in time to see the bat creature kill the other Weevil. She chases it to the parking garage, where it vanishes in a blaze of light. Tosh arrives on the scene, and once they've ensured that Megan is all right and spread rumours that the attackers were skinheads with knives, Tosh offers to take over so Gwen can try to enjoy the rest of what should have been her day off. Gwen returns home, but when Rhys accidentally mentions that he was at Pendefig, she loses her temper and throws the pack of MonstaQuest cards at him. She calms down and explains that she was upset by the thought that Rhys could have been killed during the incident there, but as she helps him to pick up the scattered cards, she notices the Toothsome and realises that it really is a Weevil. No one outside Torchwood should know of the Weevils' existence, so how did one appear as a fictional monster on a pack of playing cards?
Owen has been investigating a rash of skin complaints originating from the Withington Hotel, and has determined that they're the result of a mundane skin infection -- being spread by a Vredosian chambermaid posing as an Eastern European immigrant. The bus crashes outside as Owen is preparing to take her in, and he lets her off with a warning and goes to help the injured. One of the passengers, a student named Alwyn who was on his way to the MonstaQuest tournament at Pendefig Mall, is in a state of shock, and Owen has to pry the playing cards out of his hand and force him out of the bus with his friends. Shona Bolton has been fatally injured, and when Owen picks up her phone, he's surprised to hear Tosh on the other end before the line suddenly goes dead. The Weevil is still trapped at the front of the bus, but it becomes cowed and tame when Owen approaches; however, emergency vehicles then arrive on the scene, and it becomes spooked and bolts. Owen then notices a MonstaQuest advertising poster over the wreck of Wendleby's storefront, compares the Toothsome on the poster to the card he confiscated from Alwyn, and comes to the same conclusions as Gwen. When he tries to call Tosh, however, he gets a recorded message from a company called Achenbrite apologising for the inconvenience.
Posing as a security consultant from head office, Tosh convinces mall manager Lloyd Maddock to show her the CCTV footage of the skinhead attack. On their way to the security suite, however, Tosh finds Jenny Bolton's savaged body, and when she picks up Jenny's mobile, she is surprised to hear Owen on the other end before the line suddenly goes dead. Maddock leads Tosh to the security suite, and faints when he sees that something has slaughtered the two guards on duty and smashed its way out of the office. Tosh follows the trail of damage up to the roof, where she sees the bat creature that Gwen saw earlier preparing to launch itself at the shoppers milling about in the parking lot below. She attracts its attention by shooting at it, but it overpowers her before she can kill it and knocks her senseless. As she passes out, men wearing boiler suits marked with an Achenbrite company logo arrive on the roof and spray the bat creature with something that causes it to shrink to a safely containable size.
Gwen returns to the mall with Rhys, and finds Tosh suffering from a mild concussion. Tosh takes Gwen to the wreckage of the security suite, where they discover that the scattered plants that Tosh thought were decorations are in fact alien and are growing out of the shattered monitors. They piece the events together, and realise that the bat creature was somehow transmitted into the suite through the CCTV cameras in the parking garage. Tosh returns to the Hub while Gwen and Rhys question Dillon, the owner of the games shop; he is in a foul mood, as his partner Gareth, the man who designed MonstaQuest, has just walked out on him to strike a more lucrative deal with Wendleby's department store. As Dillon raves angrily, a nearby MonstaQuest card depicting a fire monster bursts into flame, and the fire monster itself emerges from the card to attack them. The monster burns away to nothing within seconds, and Gwen and Rhys get the shaken Dillon out to safety as the rest of the shop goes up. They promise to tell the police that the fire was an accident and that Dillon didn't burn it down for the insurance -- on condition that he tell them how to contact Gareth.
Jack and Ianto have gone to Torlannau Zoological Park, partly to investigate further Rift activity and partly as a date. As they pass an elderly man named Walter and his wife, they hear screaming from the big cats enclosure, and investigate to find that a Bengal tiger has apparently escaped and mauled keeper Malcolm Berkeley to death. However, the enclosure is soaked as if by a torrential rain, and a keeper named Bethan claims that the concrete wall that separates the enclosure from the visitors' area vanished into thin air and then returned. Ianto and Jack see a faint crack in the wall, as if it's been replaced in a slightly different position, and find an alien device buried in the mud. The escaped tiger then charges past them and knocks itself senseless on the glass, trying to get away from two-headed dragon that really killed Malcolm. Jack identifies the monster as a Brakkannee; it has already killed Walter, and it mauls Jack when he tries to distract it. Men wearing Achenbrite boiler suits then arrive on the scene and incapacitate it with their shrinking spray. Ianto tries to recover the alien device while the strangers are distracted, but one of the men sees him and runs forward, warning him that the defence system is still active. He is too late, and Ianto is knocked senseless when the device explodes. Neither he nor Jack yet know that these events were caused by Gareth, who used a set of MonstaQuest cards to exact revenge on Malcolm for bullying him during his teenage work experience at the zoo.
Gwen and Rhys follow Dillon's directions to a house in Rhiwbina, but arrive to find the house on fire. Nosy neighbour Mrs Stackpole claims that the house was struck by lightning while Gareth was arguing with his now presumably late girlfriend, but falls silent in shock when a three-eyed alien diplodocus bursts out of the burning house. Gwen uses her mobile to send a photo of the creature to Tosh, but the creature vanishes into thin air as she does so. Gareth then drives out of the garage in a red Ford Mondeo, and Gwen and Rhys pursue him in Rhys's company Vectra. Gareth speeds away, heedless of the collateral damage he's inflicting on the way, and just as Gwen catches up to him, he holds up a MonstaQuest card, causing the temperature about his car to plummet. A lorry skids across the suddenly icy road and nearly hits the Vectra, but Gwen drives off the road and down onto the river, which has frozen over. Gareth's car makes it across the river, but Gwen crashes into a riverboat, and her car is then attacked by a monstrous alien dog called a Mahalta. Gwen shoots it in the head, and Rhys points out that it appears on a MonstaQuest card as an Antebellum -- and that other cards in the pack include Element Cards such as lightning, fog, and ice. The sudden cold spell just as suddenly wears off, and Gwen and Rhys run for the river bank as the ice melts and the Vectra sinks into the river. Meanwhile, back in the Hub, Tosh is forced to break out a Jamaleon lance from the armoury in order to flash-fry the three-eyed alien diplodocus that materialised out of nowhere when Gwen phoned her. The close call gets her thinking about her own mortality, and she later records a message to her teammates to be played back in case of her death.
Owen heads to the zoo, pulls rank on the paramedics, and sits with Jack until he revives. Jack's leg was nearly severed and is still reattaching itself, and the Achenbrite man who confronted Ianto was apparently cut in half by the explosion. However, when Owen examines the body more closely, he discovers that its lower half is in fact just invisible. Owen arranges for the body to be sent to the Hub, while Tosh scans recent CCTV footage from the area and discovers that Achenbrite recently used its spray to round up a pack of dogs that had escaped from a breeding kennel. David Brigstocke then confronts Owen and Jack, having followed Jack and Ianto to the zoo from Holy Innocents; he witnessed the dragon attack, and, convinced that Torchwood is responsible, sarcastically asks if they're keeping a flying unicorn here as well. He refuses to leave them alone, but a policeman behind him reacts as if struck, and as he and his colleagues turn on the protesting Brigstocke, Jack's wheelchair suddenly rolls away as if by itself. A moment later, somebody kisses him, which is how Ianto manages to convey to Jack that he's survived the explosion -- and has become invisible.
Owen examines Ianto as best he can back at the Hub, but, in revenge for all of the "dead" jokes that Ianto has made, doesn't let him know that he can get dressed until the invisible Ianto has walked naked around the Hub for several minutes. Gwen returns to the Hub without Rhys, who has gone home for a shower, and the team convenes in the boardroom to discuss the events of the day. Tosh theorises a connection between the list of MonstaQuest enemies and the list of alien creatures in a device the team recently recovered from an archaeological dig in Twnpath Castle in Rhiwbina -- near Gareth's home. They have named it the Vandrogonite Visualiser, since they found it in the possession of a Vandrogon; it can be accessed telepathically and forms an empathic link with its operator, and the MonstaQuest cards seem to become active when somebody nearby is in a heightened emotional state. Achenbrite turns out to be a shell company that was once partly owned by the late Henry John Parker; its head offices are suspiciously devoid of even background Rift activity, as if it's shielded from the Hub's sensors; and its CEO, Jennifer Portland, is the mysterious Gareth's mother. When Tosh looks into Gareth's background, she discovers that he is connected to several people who have recently died in mysterious circumstances, such as Father Ninian and Malcolm Berkeley. If Gareth is using another Vandrogonite Visualiser, perhaps the empathic link works both ways and using the device is causing his own emotions to become stronger.
Tosh accesses Achenbrite's security by planting key-logging software on "lost" USB keys in the parking lot. However, since Achenbrite has already proven capable of blocking Torchwood's comms system, the team decides that the invisible Ianto is the only one who can get in unnoticed. Ianto strips naked and walks into Achenbrite, but finds the corridors full of drifting sand and alien flowers that spit dart-like seeds at him as he passes. Several employees have been mauled to death in a conference room, and Ianto sees a pile of human and alien bodies in a surveillance suite. Before he can enter the suite, Jennifer Portland arrives with her sons Matt and Chris, and, revealing that she can see him, she uses a PDA to restore Ianto's visibility. As he covers himself with a clipboard, she explains that she's just saved his life, as they had to pump toxic gas into the surveillance suite to deal with the attacking aliens.
The Portlands let Gwen and Owen into the building and set up a conference call with Jack, admitting that the situation has gotten out of control; however, some hostility still develops between Jennifer and Jack over which of them is in charge and which is the interloper, as Achenbrite has been collecting alien tech for some time and has successfully hidden itself from Torchwood. Jennifer explains that her troubled son Gareth stole the Visualiser and used its catalogue to create monsters for the card game he'd developed. However, it's become clear that the device is more than just a toy, and when Jennifer ordered her son to return it, he used it to set a Hoix and two Chantri Golems loose in the building, slaughtering her staff. Ianto points out that the alien flowers here and in the mall's security suite aren't part of the game, and he and Jack realise that the device isn't just a catalogue but a terraforming device that could remake the Earth into the image of another planet. Tosh then picks up a significant local radio broadcast: David Brigstocke has been assigned to do a "human interest" piece, and is reluctantly interviewing Italian supermodel Martina Baldachi, whose husband Jakob is playing football at the Millennium Stadium. Martina has just purchased a pack of MonstaQuest cards for her son from Gareth at Wendleby's department store -- and according to Tosh, the Rift activity around the store is going off the scale.
The Portlands head for Wendleby's with the Torchwood operatives. Gwen and Owen try to question Martina, who initially treats them with cold celebrity disdain -- until the undead Owen grips her face with his cold hands and demands to know where to find Gareth. However, Jennifer has already entered the store, intending to speak to her disturbed son before Torchwood can get to him. The main entrance has been blocked off due to the bus crash, but Jennifer gets past the police on duty -- PC Andy and his new partner -- by using an alien device that causes them to suffer nausea when they look in her direction. She accidentally drops the device once she gets inside the store, but continues on to the toy department. There, the bitter Gareth is sitting at his kiosk, ignored by most shoppers except for two students who try to buy the football tickets that Martina gave to him. When he refuses, they jeer at him and knock over his display. Jennifer arrives too late, and as she rushes forward, alien monsters materialise in the toy department and attack the staff and shoppers. Jennifer begs Gareth to stop this, but he looks at her with dead eyes and tells her that he's not her son any more. As he walks away, giant insects descend on Jennifer and tear her apart.
Jack and Tosh arrive at Wendleby's, where Tosh is slightly freaked out by the giant scary clowns in the Halloween display. While she parks the SUV, Jack looks for a way in -- and David Brigstocke spots him, and offers to direct him to the toy department on condition that he be allowed to tag along. They arrive to see Owen and Gwen fighting alien gorillas and giant insects. A rubbernecking shopper uses his cell phone to photograph one of the monsters, which vanishes before Owen can intervene; however, the other shoppers flee in terror when one of the insects tears apart an unfortunate sales assistant. Jack orders Brigstocke to jam the lifts so nobody else can put themselves in danger. Matt and Chris shrink the insects devouring their mother and step on them, crushing them to death, while Jack sends the last of the monsters to the Torchwood Vaults via his cell phone. While Ianto deals out retcon to the broken Portland boys, Jack tries to figure out where Gareth will be heading. He'll need a large, emotionally excited crowd to power the Visualiser -- and Brigstocke points out that Gareth now has tickets to the international match at the nearby Millennium Stadium. And the match is going to be broadcast over live TV.
Brigstocke accompanies Jack and Tosh to the Stadium, where Tosh hacks into the building's control systems to jam the entrances and retractable roof, keeping people out until Torchwood can deal with Gareth. She then uses her own Visualiser to look for the emotional energy from Gareth's -- and to defend herself from a band of rowdy hooligans with a blast of elemental wind. Inside the Stadium, they find Gareth standing on the pitch, surrounded by MonstaQuest creatures; a fire demon incinerates two of three stewards running towards Gareth, and Jack easily convinces the survivor that the police should keep everyone out until his team has dealt with the situation. Without the emotional energy of the crowd to draw on, Gareth begins drawing on the Stadium's electrical grid, and Tosh and Brigstocke head for the press gallery to cut the power. Jack tries to get close enough to Gareth to shoot him, but Gareth has now fallen completely under the machine's influence and the Stadium is being transformed into the image of another world -- complete with the alien flowers, which spit poison darts into Jack and kill him before he can react.
Tosh and Brigstocke find the journalists in the press gallery in a state of catatonic shock; they've looked into the eyes of a "Gorgon Gecko," a creature that doesn't actually exist in the catalogue but was created out of Gareth's imagination. When Gareth sends a pack of bat creatures -- Kiroptans -- to attack Tosh and Brigstocke, Tosh fights back with her own fears and imagination, using the Visualiser to create a giant scary clown that attacks Gareth. She and Gareth are effectively fighting each other in a real live game of MonstaQuest. Brigstocke helps her, using the Visualiser to create a flying unicorn based on his earlier sarcastic comment at the zoo. Jack revives, flies over the pitch on the back of the unicorn, and shoots Gareth in the head. The terraforming process has opened up a crevasse in the pitch, and Gareth's body and the monsters that he created fall into the crevasse, which closes up. Brigstocke lets go of the Visualiser to give Jack a thumbs-up, causing him to fall to another of his deaths when the unicorn blinks out of existence in mid-flight.
Later, while the rest of the team celebrates their victory, Jack takes Brigstocke out to dinner at Casa Celi and finally tells him that his friend Rhodri was killed by a Weevil. Brigstocke now understands what Torchwood does and why they cover it up, and he asks Jack to let him join the team. Jack promises to consider it if Brigstocke gives him the names of the contacts who helped him stalk Torchwood and gather information on them, and Brigstocke does so, unaware that Jack has already slipped retcon into his drink. By tomorrow, he will have forgotten that Torchwood ever existed.