Rhys books an appointment with estate agent Brian Shaw to look at a flat in SkyPoint, a state-of-the-art apartment complex by the Bay. Gwen is concerned by the price of the flat and by the fact that Besnik Lucca, a suspected criminal kingpin, lives in the penthouse suite; however, Rhys admits that he knows they can't afford the flat yet and is just imagining a future in which they can. Gwen is touched by his enthusiasm, at least until she and Rhys follow Shaw into the bathroom to find that he's vanished into thin air. There is no other exit from the bathroom, and they heard no signs of a scuffle. Gwen reports the incident to her teammates, and Tosh scans the complex but finds no sign of Rift activity. She and Jack break in to investigate, unaware that they're being watched...
SkyPoint isn't just Besnik Lucca's home, it's his private fortress, and he has CCTV cameras installed everywhere in the building, including the other flats. He returns home after supervising the murder of an 18-year-old runner who cut coke with flour and didn't let Lucca in on his increased profits, and relaxes by watching what the other tenants are up to. His accountant Ewan Lloyd lives in the building with his beautiful wife Wendy and young daughter Alison; however, Wendy isn't changing clothes at the moment, and Alison is just playing with her pixie doll, Mr Pickle. Lucca then sees Jack and Tosh enter the flat from which Brian Shaw disappeared, and sends a thug to bring them to the penthouse for questioning. The thug forces Jack and Tosh to walk out of the flat ahead of him, but before he can follow them out, the door suddenly slams shut of its own accord. Jack and Tosh open the door to find that the thug has also vanished, and leave, unaware that Lucca has seen everything.
Meanwhile, Owen Harper is on a mission of his own. Unable to sleep, he's taken to walking the city at night, and one night he saw two improbably beautiful, barely legal twins walk into Constantino's coffee shop and pick up a French philosophy student named Jean-Claude Gabin. Suspicious, Owen followed them out of the coffee shop but lost them when he bumped into a drunk at the door; by the time he caught up to the twins, they'd already taken Gabin into an alley and torn him to shreds. Owen watched from hiding as the twins devoured every scrap of meat and bone on Gabin's body, tossed his clothing into a dumpster, and walked away, giggling. He has returned to the coffee shop every night since, hoping that the twins will return to their regular hunting grounds -- but he hasn't told the other operatives.
The next day, Gwen and Jack visit the estate agents, Goldman and Grace, and ask to speak to Brian Shaw. Grace claims that Shaw has taken some time off to deal with a drinking problem, and doesn't back down when Jack catches him out in a lie. Jack and Gwen leave, but the junior partner follows them out and admits that Brian isn't the only person to have vanished from SkyPoint; however, Besnik Lucca invested heavily in the apartment, and is covering up the disappearances. Jack and Gwen return to the Hub, but Tosh's research has turned up no leads; to find out anything more, they'll have to go in undercover. Tosh and Owen thus move into SkyPoint, posing as a married couple, and as they unpack, Wendy Lloyd stops by with Alison to introduce herself and invite the newcomers to a party with their neighbours. Owen is irritated when Tosh accepts the invitation, but when he tries to remind her that they're here on a mission, she angrily points out that they need to blend in -- and that a sham marriage with him is the last thing she wanted.
Though ashamed, Owen makes a joke out of his apology, and Tosh leaves the flat, intending to let him stew for a while before accepting. She bypasses the lift's security lock and takes it to the cellar to plant a Rift activity monitor in the janitor's closet, but while there, she hears movement in the air ducts -- and when she investigates, she finds a mass of gelatinous mush containing some recognisable human body parts. She takes a sample and returns to the lift, but is caught by Besnik Lucca. She tries to bluff her way past, claiming that she works in security and bypassed the lock in order to have a quiet smoke in the cellar. Lucca seems to accept this, and invites her up to the penthouse to speak further. Tosh manages to resist his overwhelming charisma, and although she's not sure how he's going to react for a moment, he stops the lift on the 13th floor and lets her return to her own flat.
Owen has gone looking for Tosh and has found his way to the indoor park on the 24th floor, where young Alison Lloyd is reading the tale of Rapunzel to Mr Pickle. There are no other children her age in SkyPoint, and she sometimes goes exploring by herself through the building's air ducts. She innocently tells Owen that she was killed by a car, and Owen understands that although she was brought back to life on the operating table, she's seen the darkness on the other side, just like him. Wendy arrives and ushers her daughter back home, and Owen also returns to his flat, where Tosh tells him what happened to her. They return to the Hub, where Owen analyses the residue from the duct and confirms his suspicions: something has eaten the missing people, including Brian Shaw, and excreted their remains. Until they know exactly what the creature is and what it's capable of, they must treat their neighbours at SkyPoint as potential shape-shifting suspects.
That evening, Tosh and Owen attend the Lloyds' party, after a moment of embarrassment when Owen catches Tosh changing. Their neighbours include a photographer, Alun Griffiths, and his bimbo model girlfriend, Julie Jones; gay couple Andrew and Simon Taylor; the prim, librarian-like PA Marion Blake; and lawyers Mark and Roslyn Bridges. Andrew is into conspiracy theories, but Simon believes that the missing people simply skipped out on their mortgage payments. Owen chats with Wendy, who reveals that she used to believe in God until her daughter came back from the dead claiming that there was nothing on the other side. Owen then sees Tosh chatting to Lucca and ushers her out of the party, partly out of jealousy and partly out of concern. Their argument is interrupted when they see that their flat has been ransacked in their absence and that Tosh's laptop has been stolen. Furious, she storms out of the flat to confront Lucca in his penthouse suite -- and before Owen can follow, something not entirely solid, liquid, or gaseous emerges from the walls and attacks him. Unaware of this, Tosh takes the lift up the 25th floor, but finds herself stuck when the doors refuse to open. Lucca contacts Tosh and orders her to disarm, and when she reluctantly does so and emerges into the suite, Lucca reveals that he knows who she and Owen really are. Tosh tells him that Torchwood isn't interested in his criminal activities, but Lucca has his thugs restrain her nonetheless.
Owen awakens to find that Jack, Gwen, and Ianto have broken into the building to find out why he and Tosh failed to report in, and concludes that the monster rejected him because his flesh is dead. A man named Ryan then runs out of a neighbouring flat, screaming that the thing in the walls took his wife Gillian. Jack pulls the fire alarm and orders Owen and Ianto to evacuate everyone from the building. Ianto takes Ryan and the Taylors down in the lift, while Owen escorts the Lloyds down the stairs. But the building suddenly switches to emergency lighting, the lift stops between floor, and every door in the building locks itself. Lucca contacts the Torchwood operatives via their earpieces and tells them that he's decided to use them to test his building's security system. He always intended SkyPoint to be his personal fortress, he wants to know if it can withstand a determined assault -- and he doesn't care how many of the other tenants are killed in the process.
Ewan Lloyd twists his ankle when the lights go out, and Owen must take him and his family to the nearest occupied flat. Marion Blake is surprised to find Owen at her door, and he's surprised to find that she's not a prim and proper PA at all but a dominatrix who was waiting for a client to turn up. Owen mixes together some of Marion's household chemicals to create small explosives that he can use to break through the doors in the building, while Ewan retreats to the bathroom -- where he calls Lucca on his mobile and begs to be let out of the building. Some time ago, Ewan noticed discrepancies in SkyPoint's finances, but then his daughter was struck by a car and came back to life claiming to have seen nothing on the other side, and his wife had a nervous breakdown, convinced that her daughter had been possessed by a demon. Unable to cope, Ewan hit the bottle hard and never mentioned what he'd found -- and Lucca contacted him at his lowest ebb to thank him for his discretion. Ever since then, Ewan has been complicit in Lucca's criminal activities. Nevertheless, Lucca is offended by Ewan's assumption that he owes him anything, and only agrees to let Alison and Wendy out of the building on one condition.
Ianto is stuck in the lift with Ryan and the Taylors, trying to evade their questions about what he does for a living, when the gelatinous creature seeps in through the ceiling. Ianto shoots at it to no avail, and tries to get between it and the others; however, Ryan charges the monster, screaming his wife's name, and the creature consumes him and oozes back out of the lift. Ianto easily convinces the Taylors that the time has come for them to get out of the lift, and the matter becomes more urgent when he sees that a stray bullet has hit the lift cable, causing it to fray. He and the Taylors climb through the lift's inspection hatch, pry open the sixth-floor doors, and escape moments before the cable snaps and the lift plummets to the bottom of the shaft.
Ewan tries to throttle Owen to death while Owen is splinting his twisted ankle, but Owen easily overpowers him and Ewan tearfully explains that Lucca promised to release Alison and Wendy if he killed Owen. Wendy then runs out of the bedroom, screaming, claiming that Alison has disappeared; fortunately, Owen notices that her Mr Pickle doll has gone too, and a closer inspection of the bedroom reveals an open air duct beneath the bed. Alison hasn't been consumed, she's just gone exploring again. Wendy insists upon accompanying Owen to look for her daughter, and they first check the Lloyds' flat. It's empty, but before they can leave, the monster emerges from the walls and attacks Wendy. Owen pushes her to the ground and shields her with his own body, and the creature retreats, again rejecting Owen's death flesh. Meanwhile, Lucca is watching everything on the surveillance monitors in his penthouse. Tosh, tied to a chair, tries to convince him that she'll do anything to save herself; however, he believes that the creature won't harm him, and doesn't believe that Torchwood has anything to offer him.
Jack and Gwen run into some difficulty on the 20th floor when Gwen spots a Weevil lurking in the shadows and begins shooting at it. Jack realises almost too late that Lucca has pumped psychotropic gas into the air, and when the paranoid Gwen begins shooting at him as well, he shoots out the window and forces her to breathe fresh air until her head has cleared. Lucca has also electrified the door to the indoor park, and Jack is thus killed twice: once by accident, and once deliberately opening the door so Gwen can get through safely. Once he's revived again, he and Gwen shoot out the window and carefully inch along the 15-centimetre-thick outside ledge, 24 storeys above the ground. A sharp breeze is blowing off the Bay, a flock of annoyed seagulls are unaccustomed to having humans on their territory, and two of Lucca's thugs are getting some fresh air on the balcony directly above them; nevertheless, Gwen and Jack negotiate all these obstacles and reach a lightning conductor that leads from the ledge up to Lucca's balcony.
Owen and Wendy hear loud music on the 19th floor, and investigate to find Alun Griffiths and Julie Jones shooting up on heroin and oblivious to anything that's happening outside the flat. Owen takes one of Julie's syringes and locks himself in the bathroom for a few minutes, but then Lucca's thugs arrive and take him and Wendy up to see Lucca. Lucca threatens to kill Tosh unless Owen explains why the creature didn't kill him, and Owen shows Lucca the bullet wound in his chest and explains that he's already dead. Jack and Gwen then burst in, having dealt with the two thugs on the balcony -- but Alison then enters the suite, holding Mr Pickle and claiming that Mr Lucca is her friend. Lucca reveals that he made friends with Alison for his own safety, because Mr Pickle is the monster. Wendy thought it was a doll from the hospital's lost and found, but in fact it's a manifest thought-form that Alison inadvertently brought back from the other side with her, and it's been consuming people for the cellular material it needs to retain corporeal form. It stepped up its attacks tonight because it perceived Torchwood as a threat, and Lucca now orders it to dispose of that threat. Mr Pickle transforms into the gelatinous creature and attacks, but Owen leaps forward and injects the creature with the contents of the syringe -- his own dead blood. The creature dissipates, unable to process the dead cellular material that is now inside it. Alison collapses when her bond with the creature is broken, but Tosh helps Owen to apply CPR, and they revive the young girl and reunite her with her mother once again.
Jack and his team restrain Lucca and drop him off at a police station, along with enough evidence of his crimes to keep him locked away for fifty years. Owen then goes out for a walk, telling his friends that he needs to clear his head. In fact, he returns to Constantino's coffee shop, but arrives too late to save the man behind the counter, who has become the twins' next victim. Part of Owen is tempted to let them devour him as well, in the hope that he will die for good if his body is completely consumed; however, he instead shoots them both dead. He then leaves the coffee shop to find that Tosh has followed him; he doesn't tell her exactly what he was up to, but does assure her that this is his life now, and that he has no desire to return to the darkness.