The Doctor is following a capsule through the time/space vortex, and informs Rose that it's emitting a distress signal. He locks onto it and it jumps time tracks and heads toward the center of London. They materialize in an alleyway in London and set out to find the capsule, although the Doctor notes they may be several weeks or months behind. As they start searching, they're unaware that someone is watching them. The pair finds a locked door with music behind it. As the Doctor opens the door, Rose hears a child's voice, calling for its mummy. She looks up at the roof and sees a boy wearing a World War II gas mask, and goes off to investigate.
The Doctor determines that the music is coming from a night club and goes inside.
Rose runs back to the TARDIS and up the stairs to approach the child.
The Doctor gets up on stage and asks the audience if they've seen things dropping from the sky. The audience reacts with amusement.
Rose approaches the boy and tells him not to move. A rope drops down and she starts to climb up.
Air raid sirens go off and the audience quickly leaves, and the Doctor realizes it's World War II.
Rose keeps climbing only to discover that she's grasped a barrage balloon's anchor line. It lifts up, taking her with her above the London cityscape as German bombers fly overhead.
The Doctor runs out to find Rose and hears the phone on the TARDIS exterior ring. As he prepares to answer it, a young girl, Nancy, appears and tells him that it's not for him. She refuses to explain and he asks how it can be ringing. When he turns back to question her further, he discovers that she's vanished. The Doctor then picks up the phone and hears a boy's voice at the other end, calling for his mummy and asking if he's his mummy. The boy ignores the Doctor's questions, and the Doctor hears a noise nearby. He runs to investigate and spots a woman ushering her family into a bomb shelter. Once they're inside, Nancy emerges from the shadows and goes into the house to take their food.
Rose continues to hang on for her life as planes fly all around her. On the balcony of a building at a flyer's club, Captain Jack Harkness spots Rose using futuristic binoculars. He informs his comrade that he has to meet a girl.
Nancy finds a feast laid out on the table and whistles for the other local children. They come in and feed themselves.
Rose loses her grip but a beam of light shines out, suspending her in mid air. Captain Jack announces that he has her. Using the controls in his ship, he lowers her down via a tractor beam and she faints from the shock.
As more children run to the house, the Child emerges from the shadows and watches from outside. Inside, Nancy feeds all of the children who have arrived. The Doctor arrives and serves himself, and Nancy tells the children to stay where they are. He befriends them and determines that they came back because they weren't being fed properly on the farms they were sent to. Nancy wonders he wants and he asks her about the phone, but she refuses to answer further. The Doctor asks after Rose but no one has seen her, and then asks about the capsule. When he shows them a sketch, Nancy appears to recognize it. Before she can respond, there's a knock on the door and the Child calls for his mummy. Nancy realizes the door is open and rushes to lock it before the Child can get in. The Doctor asks for an explanation and Nancy says that the Child isn't exactly a child. She tells the other children to get out the back, while the Doctor approaches the front door and watches as the Child reaches through the mail slot. Nancy warns the Doctor that if the Child touches someone, he makes them empty just like him. The phone rings and Nancy says that the Child can make phones ring. The Doctor answers it and hears the Child asking for his mummy. The radio turns on and a child's toy starts chanting "Mummy" over and over. The Doctor notices a scar on the Child's hand and tries to talk to him, insisting there's no mummy there. The Child seems to understand what he's saying but continues to ask to come in. The Doctor opens the door but the Child has disappeared.
Rose wakes up and Captain Jack claims to be with the Royal Air Force as an American volunteer. However, she realizes that he's using psychic paper and he readily admits that he's not who he claims. Captain Jack also figures she's not native to the time period. He offers to treat he hands for the rope burns using nanogenes and says that she must be a Time Agent. Captain Jack then offers her a drink on the balcony and takes her out onto the roof of his cloaked spaceship, hovering in front of Big Ben. He uncloaks the ship and pours champagne for the two of them.
Nancy runs off to a rail yard where she hides some of her food. The Doctor finds her and explains that what he's looking for arrived a month ago, the same time that the Child started stalking Nancy and the other children. Nancy admits that she saw a "bomb" fall near Limehouse Green Station, but now the soldiers are guarding it. The Doctor wants her to take him there, and she says that he needs to speak to the doctor.
Jack asks if Rose is authorized to negotiate for the Time Agency and says he has something to offer. When she says she has to talk to her companion, he wonders how disappointed he should be. Jack then turns on Glenn Miller music and invites Rose to dance. As they dance, Jack says that he landed a Chula warship in London, but in two hours a German bomb will destroy it. He asks what Rose is willing to pay for it, When queried, he admits that he's a freelance criminal. Jack wants to talk to her companion and offers to easily locate the Doctor by scanning for alien technology.
Nancy leads the Doctor to the crater where the capsule landed, and the nearby Albion Hospital where "the doctor" is. She insists that he should talk to the doctor first, saying that once he does, he won't want to get inside. Nancy then starts to go, and the Doctor asks her who she lost. The girl admits she lost her little brother, Jamie, after she went for food. He followed her despite the danger and was caught in an air raid. As she leaves, she's unaware that someone is following her. The Doctor goes to the hospital gate and breaks in,
Inside, the Doctor finds the patient ward, the beds filled with people all in the same stiff, unnatural pose. Moving on, the Doctor finds another ward filled with patients. each one is in a similar position and wearing gas masks. Dr. Constantine comes in and explains that the ward is filled with patients in a similar condition. However, he says that they're not wearing gas masks. Constantine coughs and admits that he's dying, and then tells the Doctor not to touch the flesh of any of the patients. The Doctor examines them and determines that each one has massive head trauma, collapse of the chest cavity, a scar on their left hand, and a gas mask fused to their flesh. Constantine glances at his own left hand, which has the same scar. He then explains that initially there was only one patient. Everyone who touched the patient was "infected," and they spread the infection until everyone in the hospital had the same symptoms. Further, there was no cause of death, and none of the patients are dead. Constantine slams his cane on the floor, and all of the patients sit up. He explains that he's stayed to care for them a best he can, but the authorities plan to blow up the hospital and blame it on a German bomb. However, Constantine knows that it's too late and other cases have broken out. He chokes further and tells the Doctor to go to Group 802 and find the first victim, and that he was Jamie, Nancy's brother. The doctor tells the Doctor to seek out Nancy and find out what else she knows, and then clutches at his throat and says "mummy." The Doctor watches as Constantine's face transforms into a gas mask.
Jack and Rose arrive at the hospital and Jack introduces himself. Rose has the Doctor play along with their assumed role of Time Agents and asks him in private what a Chula warship is.
Nancy returns to the home to get more food, and the Child speaks over the radio. She sees him lurking near the stairs and hides under the table.
Jack has no idea what has happened to the patients. The Doctor wants to know about the Chula warship, but Jack insists it has nothing to do with the patients. Finally he admits that the Chula "warship" is an ambulance, and he threw the empty capsule at them to get their attention so he could negotiate. Once they arrived, he planned to destroy it before they could figure he was conning them. Jack realizes that they're not Time Agents but insists he's not to blame. The Doctor explains that some kind of virus is converting human beings into the mutated creatures, but he has no idea why.
The Child enters the dining room and walks toward the table. He bends over to pick up an apple and Nancy tries to run out the door, but the Child closes it with a gesture and asks if she's his mummy.
Rose examines one of the patients, but they all sit up and start chanting, asking about their mummy.
Nancy tries to get through to her brother, but he approaches her.
The Doctor warns Rose and Jack not to let anyone touch them or they'll be transformed the same way. The patients close in on the trio, chanting.