In a meeting room in a posh golf resort, Frankie Ellish is delivering bad news to Robertson about something which could be potentially damaging to him and his business. Robertson's assistant, Kevin, tries to hurry him to his waiting plane. Robertson tells Frankie to make the problem go away.
Najia comes into the room unexpectedly and Robertson is upset to have her see him. She's going to be the general manager of the resort. He fires her, tells Frankie she has one hour to fix things, and leaves with Kevin.
The Doctor and Companions are having issues landing the T.A.R.D.I.S. but manage to arrive in Sheffield only a half-hour after they left from the warehouse. The Doctor begins to make her goodbyes to the Companions but is visibly sad to head off on her own and jumps at Yaz's invitation for tea. Ryan joins them but Graham heads home for a bit on his own.
As they approach Yaz's flat, The Doctor notices a woman in the hall and asks if she's okay. The woman, Jade McIntyre, says she's fine. The Doctor notices a spider web in the corner and then follows Yaz and Ryan.
Once in the flat, Yaz's father is excited she's brought friends home. Her sister, Sonya, doesn't believe she has friends.
The Doctor tries to make small talk and asks about the collection of used jugs and containers piled up in the flat. Sonya and Yaz roll their eyes at their dad's belief that the junk is evidence of a conspiracy. Sonya interrogates Ryan on his relationship with Yaz and seems to be flirting.
Yaz gets a call from her mother, Najia, who needs a pick-up from work, upset over being fired. Yaz reluctantly agrees, not knowing what's going on with her. Yaz leaves after warning The Doctor and Ryan that her father's pakori is terrible.
Graham's standing outside the home he shared with Grace, sad to go in. There's a spider web in the shot when he enters the house. He hears Grace's voice and sees her giving him advice about how to carry on.
The Doctor offers to pick up a package for Hakim from the flat a couple of doors down where Dr. McIntyre is still trying to get a response from Anna, the flat's occupant.
Dr. McIntyre tells them she worked with Anna at the university and she hasn't been in and hasn't called in sick. The Doctor calls in through the mail slot and there's no response. She asks if Jade is really worried about Anna and offers to "sort the lock" and open the door.
The flat is covered in webs and the power is out. Jade says it has only been a few days since she'd seen Anna. While they inspect the flat, back at his house, Graham is holding one of Grace's coats close to smell her scent when he hears a noise upstairs. He finds the upstairs covered in webs as well. He finds a large spider-like shape on the floor. In Anna's flat, The Doctor, Jade, and Ryan find Anna completed wrapped in webs in her bed.
They find a giant spider under the bed. The three run out the door, shutting the spider in the room. The Doctor tells Ryan to keep it in the room and he thinks it's contained until it starts to scramble its legs under the door. The Doctor draws a line over the walls, ceiling, and floor with garlic paste and vinegar to keep it at bay.
Outside the flat, Graham comes running up to tell Ryan and The Doctor that he found a giant spider shell in the loft. The Doctor questions Jade because she didn't look surprised to see the webs. Jade admits that there's something weird going on with the spiders in Sheffield, that they're out of control.
Hakim and Sonya are waiting in their flat with the food.
Jade leads the group back to her lab at the university. On the way, Graham gives Ryan an envelope he found in the house with his father's handwriting on it. Ryan puts it in his pocket without opening it.
Frankie enters a door marked top secret in the basement corridors of the golf resort. She begins recording a video as an "insurance policy" as she walks into a space covered in webs. She is startled by something and drops her phone as she is dragged off suddenly and a clicking noise is heard.
Yaz arrives at the hotel to pick up her mom but when Najia tells her that she's been fired, they are interrupted by Kevin and Robertson. Kevin has a gun out and aimed at the women. Robertson tells Najia that the rooms of the hotel are disgraceful and offers to show Yaz how badly her mother does her job.
At her lab, Dr. McIntyre explains her spider research. She also shows The Doctor a board that tracks unusual spider activity in Sheffield over the last three months. McIntyre isn't sure how the activity is related as it spans spider varieties as well as different actions. The Doctor connects the points of activity and realizes the pattern points to a central point, possibly the spider message to others.
The point turns out to be Robertson's hotel resort. At the resort, Robertson shows Najia and Yaz a web-covered room and asks for an explanation which Najia doesn't have. He leaves for a scheduled bathroom break. The women are left alone and they hear a strange scrambling noise. Yaz presses her ear up against the wall and, in a cutaway, another giant spider is seen inside the wall, on the other side. Najia's phone rings and it's The Doctor who has arrived at the hotel and wants Yaz to let her in.
Robertson calls and leaves a message for Frankie while finishing up in the bathroom. He hears a noise in the tub and peeks down the drain. As he walks away the tub explodes and an even more giant spider climbs out. He screams for Kevin who comes in. Robertson runs out the door and closes the door, trapping Kevin in the bathroom with the spider.
The Doctor, Dr. McIntyre, Ryan, and Graham join Najia and Yaz in the hotel lobby. They hear gunshots. Robertson opens the door to check on Kevin and sees the spider wrapping Kevin up in webs. He shuts the door again then realizes that he's left his cel in the bathroom. Kevin screams out again as he is dragged down the tub hole.
Robertson runs out into the corridor where he runs in The Doctor and her group. She flashes her psychic paper at him and identifies the group as crisis investigators. He gives her the facts about the spider abducting his bodyguard.
The Doctor and Yaz go into the bathroom to investigate. The Doctor scans the tub hole with her sonic and then lowers herself down into the hole where the spider is waiting. She gets up and she and Yaz run out of the suite.
Trying to escape, they discover the entrance is completely blocked with webbing. Robertson is in denial and thinks this is all an eco-protest. They head for the kitchen to regroup. The Doctor doesn't know Robertson so he introduces himself as a big-deal businessman. Graham says he'll run against Trump in 2020. Turns out he hates spiders.
The Doctor's plan involves plans of the hotel and a captive spider. Ryan and Graham manage to capture one but they are immediately accosted by a horde of other spiders.
The Doctor, Yaz, Najia, and Robertson look over the plans for the hotel. Yaz and Najia begin to quarrel. Robertson brags about how the hotel was a repurposed venue but doesn't know what the land was before he started building his hotel. Najia informs them it was coal mines.
Jade tells Ryan and Graham that they need to go back out to the spiders and find out if the specimen they captured is the largest one out there.
As The Doctor is about to enter the top-secret room Frankie was last seen entering, Jade runs up with information. The Doctor insists they walk and talk. As they continue down the old mine paths, they find the bodies of Frankie and Kevin hanging in web cocoons. The group continues on, leaving Robertson behind. He sees Kevin's gun on the ground and pockets it.
The group finds a giant landfill around the corner. Robertson admits one of his other businesses uses the old mine spaces to dispose of wastes. They realize that all the different wastes, in combination with the naturally-occurring mine gasses, has created a soup of toxic waste which is affecting the spider population.
Ryan and Graham are in the first meeting room, looking for spiders, and Ryan discloses that he read his father's letter, that his father wants Ryan to go live with him. Ryan doesn't like how his father refers to Graham. Graham asks Ryan if he checked the ceiling when they came in. They turn their flashlights to the ceiling and an enormous spider is waiting there. It lands on the floor and chases them out of the room.
The group meets up in the kitchen and realize that the biggest spider must be the mother spider and the other ones are babies, some of which went into the city to hunt. The Doctor tells Robertson to show them his panic room.
Using the sound system that Robertson had installed in his panic room, they lure the spiders using a cranked dance radio station. With all the smaller spiders in the panic room, they approach the mother spider armed with a diluted peppermint and tea tree oil concoction.
As they approach, Dr. McIntyre notices the spider's behavior and realizes that she's grown too big to be able to breathe efficiently. She's dying. Robertson walks in and shoots the spider.
Back at Graham's house, Ryan heads out, leaving Graham alone. He glimpses Grace again but doesn't speak.
Over at Yaz's flat, Hakim is clearing away the waste containers he'd collected. They've run out of bread so Yaz offers to go out to get some.
The three Companions meet up outside the T.A.R.D.I.S. and Graham knocks. The door opens and they walk in. The Doctor thinks they're there to say goodbye for good but they've decided that they want to join her on her adventures. She warns them to be sure but is very happy to have them aboard as Team T.A.R.D.I.S.