At Sanderson & Grainger, a London department store, the lights flicker outside on the street. Inside, clerks Kelly and her supervisor Shona are closing up when the power flickers. Shona tells Kelly that she’ll clean up and Kelly thankfully leaves.
At the home of Craig and Sophie, Sophie is leaving with a friend and Craig assures her that he can cope on his own. As Sophie leaves, the power flickers.
At the department store, Shona cleans up the changing rooms and sees someone moving in one of the booths. She tells the customer that they’ll be closing in a couple of minutes.
Craig calls his mother and insists that he can handle things on his own. There’s a knock at the door and he goes to answer it, and finds his former roommate the Doctor on his doorstep. The Doctor complains that he doesn’t like the redecoration and Craig explains that they’ve moved. After an attempt at small talk, the Doctor turns and goes but notices the power flickering and realizes that something is wrong. He barges in and starts scanning, and realizes something is wrong with the house.
The lights continue to flicker as Shona picks up discarded clothing, and realizes the customer is still in the changing room.
The Doctor bursts through a closed door and finds Craig’s baby son Alfie.
Shona pulls aside the curtain and finds herself facing… a Cyberman.
As Craig takes care of Alfie, he complains that he can’t handle it and admits he can’t handle being a dad. The Doctor manages to quiet the baby as Craig explains that he wanted to prove to people that he could handle Alfie on his own. Craig wonders why he’s there and figures that something is up, but the Doctor says that he’s on a farewell tour. He glances through a newspaper and hesitates, and Craig figures that he’s noticed something. The Doctor refuses to notice it and walks out, but can’t help scanning around the area for electrical fluctuations and teleport energy.
The next day, the Doctor is busy working at the department store in the toy department and entertaining children. Craig is shopping and telling Sophie over the phone that everything is fine, and sees the Doctor. He wonders why the Doctor didn’t leave and his friend insists that he wanted to live in the moment. As the Doctor puts a toy robot dog away, something scuttles across the aisle a few feet away. He goes after it and Craig demands to know if there is something dangerous going on and if his son is at risk. The Doctor finally tells him that several people have gone missing recently. Each time there has been a power fluctuation, someone has been using a teleport relay inside of the store, and the CCTVs have been wiped. Craig points out that the council has been putting in new wiring recently.
The Doctor tries to get Craig out of the store by repairing an out-of-order lift and taking him down. As they go down, the power flickers and the Doctor realizes they’re in a spaceship. He tries to distract Craig by offering to kiss him, but Craig realizes that they’re in trouble and panics. A Cyberman approaches them and the Doctor uses his sonic screwdriver to reverse the teleport and send them back to the elevator. The Doctor fuses the relay so the Cybermen can’t use it again, and then goes outside to scan the sky to find the spaceship. It’s shielded and he’s unable to find it, and warns that fusing the relay only bought themselves some time. Craig wonders why the Cybermen haven’t invaded in force, and the Doctor tells him to take Alfie and go home. He refuses, figuring that the safest place to be is next to the Doctor because he always survives. The Doctor reluctantly agrees and they go to investigate.
Back inside the store, the Doctor hugs Craig and tells him to look around and take Alfie with him to get people’s sympathy. A clerk, Val, tells the Doctor that he and his partner look sweet, mistakenly assuming that they’re life partners. When he asks if she’s seen anything unusual, she starts spreading gossip but finally mentions a silver rat-thing.
As Craig walks through the store, a silver-rat thing crosses the floor behind him, unseen. He talks to Kelly in the lingerie section but she assumes he’s flirting with her and calls over the security guard, George.
The Doctor returns to the toy section with Val, who assumes the silver rat is some kind of toy. Meanwhile, Craig tries to impress George with how cute Alfie is and tries to get away. He knocks over a rack, attracting the Doctor’s attention. The Doctor comes over and asks if they’ve seen a silver rat, compliment George on his uniform, and assures the guard that Craig is with him. Kelly mentions that she’s doing twice the work because Shona has disappeared and the Doctor asks for more information. He then takes Craig back with him to the changing rooms and confirms that a Cyberman took Shona. The Doctor explains that the silver rat is a Cybermat, an infiltration unit that collects power. He plans to stake out the store, and Craig wonders if it’s a coincidence that aliens have shown up twice at the same time that the Doctor has. He takes Alfie to the changing station in the electrical good section while the Doctor insists that it’s all a coincidence.
As Craig goes, the Doctor sees Amy and Rory in the store and ducks out of sight so they don’t notice him. As they leave, the Doctor sees a perfume poster with Amy’s face on it, a perfume “for the girl who’s tired of waiting.”
That night, the Doctor, Craig, and Alfie hide in the store until it closes and then search for the Cybermat. Meanwhile, George checks the hallways and the lights flicker as something nearby scurries past. It runs past the Doctor, who catches it with a fishing net. He realizes that it’s on low power and they assume it’s harmless until it bares a set of organic teeth.
George hears something moving in the basement and searches, unaware that a Cyberman is closing in on him. The Doctor is trying to calm Craig down when they hear George scream. The Doctor runs to investigate and finds George’s body, and a Cyberman knocks the Time Lord unconscious. Craig finds him and wakes him up, and the Doctor explains that the Cyberman took George to the spaceship. He realizes that the Cyberman is collecting spare parts from humans and didn’t take him because his physiology is incompatible, and wonders why the Cybermen need parts. Craig insists that they need to get back to base.
At Craig’s home, Craig goes to get milk and leaves Alfie with the Doctor. When the baby starts crying, the Doctor hesitantly goes to check on him and tells Alfie that he has a normal life ahead of him. He then bemoans the fact that he’s old and near the end, and Alfie has everything ahead of him. The Doctor talks about how he dreamed of the stars when he was Alfie’s age and hopes he has as much fun as he did.
There’s a beeping noise behind him and the Doctor realizes that the Cybermat has reactivated. He stuns it with the sonic screwdriver and runs downstairs with Alfie and out the back door. However, the Doctor drops the sonic screwdriver and locks the door behind him. Meanwhile, Craig comes in with the milk, putting his cell phone down and leaving it just as the Doctor tries to call him. Craig enters the kitchen and the Cybermat attacks him, and the Doctor puts Alfie in the swing and jumps through the window to help his friend. The Doctor finally finds the right frequency to shut it down for good and congratulates Craig on holding his own against it.
The Doctor reprograms the Cybermat and explains that it was transmitting electricity up to the spaceship. He hesitates when he realizes that Craig and Alfie almost died because he was there, and admits that he should never have come there. Craig points out that the planet would be in ruins without him, and the Doctor explains that his time is running out and he’ll die the next day. He turns and discovers that Craig has dosed off with Alfie and hasn’t heard a word he’s said. The Doctor then slips out, holding the Cybermat.
Craig wakes up as Sophie calls to leave a message on the answering machine, and he finds a note from the Doctor saying he’s gone off to stop the Cybermen. He puts Alfie in the baby sling and explains that he has to help the Doctor whether he wants someone else to help him or not. At the store, the Doctor meets Val and tries to work out how the Cybermen returned to the store so quickly after he fused the teleport relay. He goes back to the changing room and finds a hidden door leading into an underground tunnel. The Doctor climbs down and finds the spaceship, buried. Going inside, he finds five inactive Cybermen
Craig runs into the store and Val tells him where the Doctor went. He has Val look after Alfie while he goes to find his friend.
A Cyberman confronts the Doctor, who works out that the spaceship crashed there centuries ago and was inactive until the council rewired the area. Now the Cybermen are accumulating enough power and parts to create a new army and conquer Earth. The Cyberman recognizes the Doctor, who gives it the choice to deactivate or he’ll do it. However, a second active Cyberman comes in and approaches the Doctor.
Craig runs back to the changing rooms and finds the tunnel. He picks up a laser scanner and goes down, entering the ship. The other Cybermen have activated and are discussing what to do with the Doctor. Craig bursts in and tells them to leave the planet or he’ll shoot them with the laser scanner. Unimpressed, they easily capture him and state that they will take his fear and make him into their Cybercontroller. The Cybermen place him in a conversion unit and the Doctor activates the Cybermat to drain their power. A Cyberman crushes it and informs the Doctor that he has failed.
As the Cybermen begin to cleanse Craig’s brain of emotion, the Doctor tells him that they can’t succeed if he fights back. He yells that he’s always believed in humans and doesn’t mind if he dies as long as Craig can prove him back. A Cyberhelmet closes over Craig’s face and full conversion begins.
Val tries to comfort a crying Alfie as the lights flicker. The Cybermen detect the baby and dismiss its fear as irrelevant. The Doctor calls out, apologizing that he can’t help the baby, and the Cybermen realizes that Craig’s emotions are reactivating as he hears his son cry. The Time Lord tells his friend that he’ll never have a better chance to prove that he’s a good father, and tells the Cybermen that they’ve made the mistake of making a daddy mad. Craig breaks free and the emotional influx induces a feedback in the Cybermen. The Doctor and Craig try to get out but discover that the ship is sealed. As the Cybermen explode, the Doctor gets Craig into the teleporter and activates it just in time as the spaceship blows up. They appear in the lift and Craig runs to his son.
Later, the Doctor explains to a confused Kelly what happened while Craig gets a new shirt. Val asks about Craig’s partner and both he and the Doctor finally work out that she thinks that they’re together. When Craig turns to have the Doctor explain, he discovers that his friend has slipped away. Craig returns home with Alfie and discovers that the Doctor is there and has cleaned everything up. He explains that he went back in time, using up most of his remaining hours, and that he did it for his mate. The Doctor then translates for Alfie, saying that he now calls him Dad and that he’s very proud of his father. Craig offers to help him but the Doctor says that no one help him. He takes a pack of Sophie’s blue envelopes and tells Craig that he’s going to America for an appointment he can’t miss. Before he goes, Craig gives him a cowboy hat from a friend’s dad. As Sophie knocks on the door, the Doctor slips away before Craig can say anything else. He lets her in and she wonders if anything happened, but Craig insists that nothing happened. However, Alfie says the word “Doctor” and Sophie realizes something is up.
The Doctor goes to the TARDIS and tells it that they’ll be making one last trip. Three neighbor children come over and he tells them that he was here to help and that they’re very welcome.
In the future on the day she becomes a doctor, River Song reads the children’s reports and checks her TARDIS diary, which gives the date of the Doctor’s death at Lake Silencio. Madame Kovarian comes in and says that River can’t remember her because they’ve thoroughly reprogrammed her brain. The Silence surround River and Kovarian congratulates her on becoming a doctor. She asks River if she knows what the date means, and River admits that it’s rumored to be the day that the Doctor dies at the hands of an impossible astronaut. Soldiers come in bearing the astronaut’s suit and Kovarian addresses River as Melody Pond, telling her that she never really escaped them. As the soldiers sedate River, Kovarian tells her that she will be the woman who kills the Doctor.
When River wakes up, she finds herself in the suit, underwater at Lake Silencio.