A flashback narrates the end of a great battle in Britain long ago where a great foe was barely defeated and the surviving war leaders divided the body of the enemy into three parts and took the pieces to three points scattered across the planet and buried them there, setting a guard to stand vigil over the pieces forever. One went to an island in the South Pacific. One went to Siberia. And the third gets killed by a robber's arrow somewhere in Yorkshire and never fulfills his mission.
In the present day, the third guardian's body is being excavated from a dig under Sheffield Town Hall. The two archeologists, Lin and Mitch, are working on New Year's Day. They kissed the night before and are discussing where their relationship is going when Lin finds something in the hands of the body they are digging up.
Mitch bags and catalogs the find and they discuss the possible identity of the body, leaving the specimen under a UV lamp which seems to activate it. In the two corresponding sites for the great enemy's body part burial, there is a disruption and the guardians dig up their piece only to see it disappear suddenly. Back in Sheffield, the specimen Lin and Mitch found has grown in size and is definitely animating.
The Doctor and the Companions are watching cosmic fireworks from the door of the T.A.R.D.I.S. as a marathon of New Year's celebrations. They debate which was the best. Just as they are about to head off to their twentieth celebration, an alarm goes off and The Doctor feels it is important enough to head back to Sheffield.
In the Sheffield excavation site, the specimen wiggles itself off the table, knocking off a bunch of bags in the process. Mitch notices that one bag is missing when picking them up and Lin heads off to see if she can find the "rat" that took off with it.
Just as she finds a large organic mass on the wall next to the aquifer, Mitch notices a sound and turns to see the T.A.R.D.I.S. appear at the other end of the cavern. The Doctor emerges, introduces everyone to Mitch, and then they go to find Lin. Lin leads them back to the creature on the wall but it's gone, leaving some residue that The Doctor takes for analysis. She places the sewers under quarantine as they lead under the entire city giving the creature free access.
Yaz escorts Mitch and Lin to the way out and gets called back by The Doctor.
Outside, Mitch offers to take Lin for a drink but she puts it off, saying she's a bit freaked out by what's happened. They part ways and Mitch sends a text when he gets to the end of the street. The text lights up on Lin's phone in her car but she's sitting at the wheel, eyes open but completely unaware.
The Doctor takes the T.A.R.D.I.S. back to Graham's home, accidentally flattening a chair when they arrive. She goes to check the goo she's collected and the doorbell rings. Graham opens the door and shuts it immediately. The doorbell rings again and Ryan goes to answer it. He comes back with a man he introduces as his father, Aaron. The Doctor points out that him not coming to Grace's funeral really let Ryan down.
While Ryan's getting his coat, Graham takes Aaron aside and warns him that Ryan's been through a lot and to tread gently.
Lin returns home and immediately heads to the bathroom. She removes her coat to reveal the organism wrapped around her body with a tentacle penetrating her spine at the base of her skull. She tries to detach it but the creature communicates with her and states that it is now her pilot.
On the T.A.R.D.I.S., The Doctor gets the results of the goo and realizes that the creature she's looking for is a Dalek.
The Dalek uses Lin to access the Internet and research the Earth's defensive capabilities. Lin tries to fight it but it warns her that if she resists, her friends will die at her hand.
Yaz tries to get in contact with Lin but gets voicemail. Lin is driving recklessly, controlled by the Dalek, and gets pulled over by the police. When the officer comes to speak to her, he is identified by the Dalek as an enemy and the tentacles emerge and pull the officer in. His partner tries to come to his assistance but Lin is out of the car by this point, kills her too, and steals her uniform and the vehicle.
Ryan and Aaron are at a coffee shop where Aaron is trying to sell an appliance to the shop owner. He sits down and they have a candid heart-to-heart.
Mitch is taking a call from Yaz when the T.A.R.D.I.S. appears right in front of him. She brings him inside. With his information of The Order of the Custodians, The Doctor is able to deduce that there's been a Dalek buried on Earth since the 9th Century.
Using Yaz's phone, The Doctor is able to track down the Dalek DNA and Lin and realizes that they're in the same place, that they're together.
Ryan arrives back at Graham's and comes into the T.A.R.D.I.S. just as the T.A.R.D.I.S.'s system blow. The Doctor realizes this Dalek is a recon scout with abilities beyond the normal Dalek, that it is possibly the first to leave Skaro. She send Graham to go get peanut butter.
Lin arrives at the MDZ archives and knocks out the security guard and uses his palm print to access the files. She burgles the place and is about to leave when the Dalek goes into some sort of spasm and The Doctor's voice is heard. They parley for long enough for The Doctor to reboot the T.A.R.D.I.S.'s navigational systems and zero in on Lin's location. The T.A.R.D.I.S. disappears and leaves Graham and Aaron back at the flat.
Mitch fills the team in on MDZ Research which buys non-terrestrial weaponry on the black market. The Doctor continues to try to track Lin but the Dalek is using its newly acquired weaponry to take down surveillance cameras. The Doctor tries to reach out to U.N.I.T. only to find out that it has been suspended due to funding shortages and Team T.A.R.D.I.S. is on its own
While Aaron and Graham are left alone, Graham brings out a tote of Grace's belongings. It's filled with souvenirs of Aaron's childhood. They discuss why he didn't attend his mother's funeral.
Lin arrives at a farm with lots of machinery parts, kills Farmer Dinkle, and proceeds to rebuilt a casing for the Dalek.
Lin tries to fight back, flinging her body into things in an attempt to injure the Dalek. The Doctor, Ryan, Mitch, and Yaz arrive. They find the farmer's body and enter the hanger. Lin is calling for help and when they find her, the Dalek has removed itself from her. The Doctor sends them back to the T.A.R.D.I.S.
She searches the hanger and a door explodes, revealing a newly built Dalek casing. She looks it over, engages in banter, identifies herself, and then it flies off to alert the Dalek fleet.
The Doctor heads back to the T.A.R.D.I.S. with the bad news. Graham calls asking for a pick-up and then lets Aaron know there's going to be a bit of a disturbance when the "cabinet" reappears. Yaz sticks her head out and says Aaron can come too.
Out near a wind farm, a military unit engages with the Dalek and gets exterminated.
The T.A.R.D.I.S. is still tracking the Dalek and when The Doctor describes the Dalek's casing as being mostly metal, Aaron realizes the microwave over he's been trying to sell can help. They strip the microwave oven's casing and remove the element.
The Doctor figures out the Dalek is heading to the Government Communications Headquarters. It breaks through to the control room and aligns all the communication transmissions to Dalek fleet frequencies. In order to send the signal, it drains all power resources, shutting down all communication in the UK including the wi-fi and Internet, forcing families to talk to each other.
The T.A.R.D.I.S. arrives in the Communications Headquarters and the team executes the plan to destroy the Dalek's casing. It works but the Dalek leaps free and takes control of Aaron. Holding him hostage, it forces The Doctor to agree to take it to the Dalek's fleet.
When the T.A.R.D.I.S. lands, Aaron/Dalek walks towards the door and The Doctor opens it on a sun going supernova, causing a vaccuum effect to suck the Dalek into its core. It nearly drags Aaron out with it but Ryan gets through to him and is able to grab hold and throw off the Dalek.
The team debriefs back in the tunnels under Sheffield and Aaron gives Ryan his blessing to continue traveling with The Doctor.
Aaron's voice narrates a bookend summary of the team's battle over the final exit by the T.A.R.D.I.S.