Ryan Sinclair is giving a Youtube testimonial to the greatest woman he ever met. Because of a dyspraxia condition, he cannot ride a bike at age nineteen despite help and encouragement from his grandmother, Grace. and her husband, Graham. In frustration, he throws his bike down a crevasse and when trying to retrieve it, he witnesses the appearance of an extraterrestrial pod. He calls the police
Yasmin Khan sorts out a dispute between two women disputing a parking issue. She complains to her supervisor that she wants to do more than parking disputes. She reports to Ryan Sinclair's call. Turns out they knew each other in grade school and catch up a bit. She thinks the pod is a prank until Ryan convinces her to touch it and feel how freezing it is.
Grace and Graham are on the train, discussing whether Ryan will ever call Graham "grandad" since they've been married three years now, and there's a crash and the lights go out. Grace goes to investigate but can't get far because the doors are locked. There's something coming down the train and she calls Ryan to tell him what's happened and then their call gets cut off. Yaz and Ryan get in the police car and race to the scene.
The "something" on the train turns out to be a moving mass of wires and mechanical bits. As it approaches the three train passengers, something ELSE falls through the roof of the train between the humans and the threat. The Doctor pops up and her attention is directed to the imminent danger. She turns, pulls another electrical cable from the roof, and zaps the thing. When she's informed of the locked doors, she reaches for her sonic screwdriver but discovers her pockets are empty.
She questions the electrical mass but it bypasses her and approaches the third passenger. The Doctor cautions them all to stay still, that the thing could've killed them already if that was its intent. Yaz and Ryan appear on the scene and the electrical entity zaps them all and escapes through the hole in the train roof.
The Doctor tells the passengers to stay put and she goes to check the rest of the train with Ryan and Yaz. They make their introductions (except that The Doctor doesn't know who she is) and continue down the train. Yaz wants to call this in but The Doctor asks her to wait until they know more.
While debriefing, The Doctor remembers the T.A.R.D.I.S. exploding and dematerializing. The other people are very confused but when Ryan explains The Doctor thinks the electrical mass is an alien, Graham and Carl scoff at the idea. Carl leaves, wanting to forget about the whole experience.
When The Doctor asks if anyone's seen anything else out of the ordinary, Ryan volunteers the information about the pod in the crevasse.
They drive to the location of the pod but it has disappeared. It's been packed into the back of a white van and moved to a garage in the town. Two young men are examining it. One of them wants to call the authorities. The other, Rahul, seems to know something about it and insists the other man leave. Once alone, he sets up a camera with a live feed and sits down to watch the pod.
The Doctor and her entourage are discussing their plans. Yaz is going to return to the police station and see if there are more reports of the pod. Ryan is going to check social media. Graham wants to ask a bus driver because he used to be one and he believes they know everything that's going on. Grace is going to confer with her nursing friends on WhatsApp.
The Doctor starts to feel faint. Grace offers to take her to the emergency room. Suddenly, The Doctor sticks her finger up her nose and predicts that she will faint almost immediately.
While Yaz and Graham are asking around, The Doctor lies unconscious on Grace's couch. She starts to glow and Grace and Ryan watch as a spark floats off of her. Grace notes that she has two separate pulses.
Rahul watches the pod crack open.
Elsewhere, the ball of electrical cables seems to be gathering energy from the city's power grid.
There's a zap through the city and The Doctor wakes up. She realizes that everyone has been tagged with DNA bombs. She uses Ryan's phone to figure out how to track the alien.
In the garage, Rahul confronts a mechanical being and asks where his sister is to which the creature responds he should ask again. The man asks again and the creature kills him.
The Doctor directs them to the garage where they find the creature as he leaves the garage. It gets away and then finds Rahul's body, covered in freeze burns, with his jaw broken, and missing a tooth. Ryan realizes that the remains of the pod are in the garage and they deduce that the being came out of the pod. Ryan thinks he may have caused the pod's appearance by touching the lights that preceded the pod's arrival.
The tracker is not working anymore. The Doctor decides to build a new sonic screwdriver.
Ryan and Yaz agree that they both believe that The Doctor is an alien. They find Rahul's files on his missing sister.
Grace and Graham discuss regeneration with The Doctor until she pushes them out of her workspace. She creates a sonic screwdriver. It's a little sparky but she's sure it'll work okay.
They watch a video of Rahul describing his sister, Asha's, disappearance.
Using the sonic, The Doctor realizes that the pod is designed for a return journey. She concludes that the two aliens are battling each other.
Graham gets a phone call from a bus driving colleague.
A drunk man in an alley runs into the garage creature and tries throwing his take-out vegetables at him. He gets killed and tooth-robbed too.
The team finds the ball of cables (thanks to Graham's bus driver friend) and they zap it to overload it. The Doctor scans it and realizes that it was gathering data. When they realize it was after Carl from the train, the garage creature finds them and there's a confrontation.
When asked about the tooth-gathering, the creature unmasks and introduces himself as a Stenza warrior called "Tim Shaw" and shows that he wears a tooth from every prey he hunts. Carl is the designated prey he needs to hunt down in order to return to his home and ascend to the position of leader.
The Doctor points out that the use of the cable ball is cheating the rules of his hunt but she lets him have it anyways. Using a short-range teleporter, he escapes them to go hunt Carl.
Carl is at work, at the top of a construction crane, and listening to self-affirmation messages when Tim Shaw appears on site, kills his supervisor, and begins climbing the crane.
The team arrives on the scene and The Doctor sends Grace and Graham off-site to clear the entire site, warning them not to return. Yaz and Ryan and The Doctor climb a different, nearby crane. They get Carl's attention and insist that he climb out of his operating booth and over the crane arms as a bridge.
The Doctor climbs the second crane and instructs Yaz and Ryan to swing the arm around so she can get Carl.
On the ground, Grace insists on coming back on site to help. Yaz figures out the crane controls but the arms end up a distance apart and Carl has to jump. When he tries to jump, Tim Shaw grabs him, mid-air. The Doctor leaps across to confront the Stenza. She demands that he release Carl or she will destroy his transport pod's return controller circuit. He reveals that the trophies are kept in stasis on their planet. He threatens to detonate the DNA bombs but she points out that'll destroy the circuit as well.
Grace and Graham get into an electrical cabinet and Grace pulls a cable out and climbs the crane with it while Graham arranges for a power surge.
The Doctor continues to offer Tim Shaw options. He asks again who she is and she is able to answer him this time because her regeneration is nearly complete. She gives him one last chance to leave while he has a choice. He chooses to detonate the bombs but doubles over in pain himself.
The Doctor reveals that her sonic was able to remove the bombs from herself and the humans and she had replanted them into the cable mass device earlier. When Tim Shaw transferred the ball's data into himself, he also absorbed the DNA bombs. As he begins to degenerate, The Doctor tosses him the return circuit and tells him to go home. Carl kicks him over the edge and the Stenza slaps the circuit onto his chest and dematerializes.
The Doctor admonishes Carl that he had no right to do that.
Down below, Grace zaps the cable ball but falls back at the last moment and dies on the ground with Graham holding her. The Doctor, Carl, Yaz, and Ryan get back down off the crane to find them there.
Returning to Ryan's YouTube testimonial, he completes his tribute to Grace and then goes out to the field with his bike, determined to ride it for her memory. He falls over and over while The Doctor watches from a distance.
At Grace's funeral, Ryan and The Doctor wait for his father who is running two hours behind. Graham delivers a eulogy.
Afterward, Graham explains he and Grace met when he was undergoing cancer treatment. The team discusses family and how to keep family close when they are taken or lost. The Doctor points out that she's been there too long and she needs to go looking for her TARDIS. Yaz convinces her to find some new clothes.
Back in the garage, she creates a tracking device to find the TARDIS and configuring the Stenza's pod to take her there. With the trio's help, she activates the pod with the sonic but ends up zapping all three of them into outer space.