Jodie Whittaker (Thirteenth Doctor)
Chris Noth – Robertson
Sharon D Clarke – Grace O'Brien
Shobna Gulati – Najia Khan
Tanya Fear – Dr Jade McIntyre
Ravin J Ganatra – Hakim Khan
Bhavnisha Parmar – Sonya Khan
Jaleh Alp – Frankie Ellish
William Meredith – Kevin
Having returned to Sheffield, the Doctor goes to meet with Yasmin's family alongside Ryan, while Graham heads home to grieve over Grace. After Yasmin leaves to pick up her mother, Najia Khan, the Doctor and Ryan encounter arachnologist Dr. Jade McIntyre while trying to meet the family's next door neighbour. Gaining entry, the group discover it's owner, McIntyre's colleague, was killed by a large spider. After Graham rejoins them upon having found something similar at his house, the group learn that McIntyre has been investigating bizarre behaviour patterns in spider ecosystems, after suspending experiments with spiders at her laboratory. The Doctor deduces that the giant spiders and behaviour patterns are linked a recently built luxury hotel complex, which Najia worked at until recently fired by its American owner Robertson.
Arriving at the hotel and joined by Robertson, Yasmin and Najia, the group learn the spiders infest the complex and sealed them in. While Ryan and Graham capture a spider for examination, the rest discover the spiders came from abandoned mine tunnels beneath the complex, discovering the bodies of Robertson's bodyguard and personal assistant. Further investigation soon reveals the tunnel were used as a dumping ground for industrial waste by one of Robertson's company. McIntyre, who revealed to the Doctor her experiments involved genetically modified spiders, realises the giant spiders are the offspring of a specimen that had been dumped by the same company, on the belief it was dead. The Doctor theorises the toxicity of the dumping ground mutated them further.
To kill them humanely, the group lure the offspring into a panic room Robertson built into the hotel, before encountering the specimen itself in the ballroom. Upon finding it, the Doctor and McIntyre realise the spider is dying from being unable to adjust to its massive size. Before the Doctor can assure it dies humanely, Robertson kills it with his bodyguard's gun, much to her disgust. With the situation resolved, Ryan, Yasmin and Graham contemplate returning to their normal lives, but decide to see more of the universe with the Doctor, much to her delight.