The Doctor is walking down a street in London, to Adipose Industries, and breaks into the building. Donna Noble also gains entrance to the building for a conference. Inside, Miss Foster is advertising their new weight-loss pill while the Doctor watches from the projection booth, pretending to be a health inspector. Both of them talk to the employees, unaware that the other is present. Miss Foster comes into the office to tell them to increase sales.
They separately track down individuals who have been taking early samples of the pills. It turns out the man the Doctor is interviewing has a nightly disturbance and has a catflap in the door from a previous owner. The Doctor wonders if something may have left. While Donna’s interview subject Stacey is preparing for a date in the next room, Donna toys with a small mechanism she picked up at Adipose. The woman’s stomach begins to contort as Miss Foster receives an activation signal and sends out a collection squad. The Doctor has a tracking device that also picks up the signal and he follows it through the streets.
In the bathroom, the woman’s weight detaches itself and transform into small blobby creatures. Donna comes to investigate but the door is locked, and Stacey disappears entirely as she transforms into several of the creatures. Donna bursts into find Stacey gone and one of the creatures waves to her from the window before dropping to the ground below. A collection squad pulls up outside and starts collecting the blob-creatures. They leave just as the Doctor and Donna arrive on the scene, again without seeing each other and missing the collection squad as it leaves.
Miss Foster concludes someone activated the signal prematurely and finds Donna on the security cameras. Donna returns home to find her mother Sylvia waiting to give her a hard time for coming in late. She tracks down her grandfather Wilfred, an amateur astronomer, who is watching the stars up the hill. Donna goes to talk with him and watch the planets, and says if he ever sees a blue box to let her know.
The next day Donna takes the family car to Adipose Industries while in the TARDIS the Doctor tracks the signal and materializes there just after she arrives. They both go in and sneak around, but Sylvia calls Donna as she’s hiding in the women’s restroom. She needs the car, and mentions a friend is using the Adipose pills. Miss Foster tracks an intruder to the restroom and her men kick open each cubicle… where they find a reporter from the press conference, Penny, and take her away.
The Doctor goes the building’s roof then takes a window cleaners’ platform down to Foster’s office window. Listening in, he hears Foster saying that their pill pulls the fat together and galvanizes it into forming a body. She shows them one of the blob-creatures, saying it’s one of her “children,” an Adipose made out of living fat. Donna and the Doctor spot each other and try to mime explanations to each other … until they realize Foster has seen them. The Doctor seals the door for Donna, who runs to the roof where he runs into the building and heads down. The guards shoot through the door and follow them as they get together and head back up. Donna explains that she investigated conspiracy theories figuring he Doctor would eventually turn up, and she now believes everything he said.
The Doctor rigs the window cleaners’ platform and starts to descend, unaware that Foster has a sonic device of her own. She takes control of the winch but the Doctor temporarily overrides it and tries to open a window. Foster cuts through one of two support cables and Donna is thrown out, only just managing to catch the loose cable. The Doctor tries to pull her to safety as Foster starts to burn through the second cable, but the Doctor overloads her sonic device with his screwdriver and then catches it as it falls. Using both devices, he manages to get the window open, goes inside, and gets to Foster’s office where Donna is dangling in front of. He gets her inside and frees Penny, but ends up confronting Foster and her guards in the employee area. Foster identifies herself as the matron commander of a nursery fleet and she’s been hired to use Earth as a breeding ground for the Adiposian First Family after they lost their breeding planet. She explains that the Adiposian children can totally destroy the host body in case of an emergency. The Doctor gives her one warning, noting it’s illegal to seed class 5 planets. She turns down her offer and he uses the two sonic devices to create an overload and escape. In response, Foster orders her people to accelerate the breeding program.
Penny stays behind to go through the files but ends up captured when Foster and her men return. The Doctor tries to access the computer core using Foster’s sonic device. In her office, Foster opens up a panel revealing their inducer device and tells her men to find and kill the Doctor. The Doctor electrifies the door frames, knocking out the guards, then goes back to work. Donna chats with him, and he says that he destroyed half of Martha’s life and Rose is still missing. She admits that she fell back into her old life without the Doctor, and wants to travel with him.
Sylvia is meeting with her friends at a bar as Foster activates the inducer. All of the overweight patrons in the bar, and people across England, start to contort in agony as the Adiposian children emerge from them. Foster cranks the system up to double strength as the Doctor tries to override the system using the activation capsule he’s stolen. Donna comes to the rescue, giving him her capsule. The Doctor shuts down the inducer but enough of the children have been spawned to attract the Adiposian First Family. Their ship arrives above London to collect the children and the Doctor and Donna hear the signal over the computer core.
Foster addresses the horde of Adiposian children filling the streets. The ship starts to beam the children up and Foster calls out, asking them to take her as well. The Doctor and Donna get to the roof and see what’s happening, and the Doctor refuses to destroy them. Donna notes that Marsha must have helped mellow him out. They see Foster floating upward and the Doctor tries to warn her, saying the First Family knows the breeding was a crime on Earth and they will kill her. Foster doesn’t believe him… and then the First Family cut off the levitation beam and Foster falls to her death. The Adiposian ship leaves while a furious Penny threatens to report the Doctor and Donna.
The Donna accompanies the Doctor to the TARDIS which is parked next to her car, and she has all her travel gear packed in the trunk. The Doctor is hesitant to take her along and he notes that the last time with Martha, it got complicated and it was his fault. He says he wants a mate, which she initially misunderstands until she figures out he means a friend. He agrees to let her come with him but she has to make a few arrangements. She calls to explain to Sylvia that she’s leaving for a while, and drops off her car keys in a bin. She tells a blonde woman to let her mother know where it is and then runs off. The blonde woman is… Rose Tyler. She considers Donna, then walks off down the street and fades away.
Donna returns to the TARDIS and the Doctor asks her where she wants to go. She gives him directions to the hill above her house. They fly overhead as her grandfather sees Donna waving to him from the open door along with the Doctor. Her grandfather laughs in triumph as the TARDIS departs.