Donna Noble finds herself regretting her decision to decline the Tenth Doctor’s invitation to travel in the TARDIS. She has started investigating conspiracy theories in the hope that she will find him again. The Doctor and Donna, neither one aware of the other’s involvement, both investigate Adipose Industries, which is marketing a special diet pill to the people of London. They find that the pills use latent body fat to parthenogenetically create small white aliens called Adipose that spawn at night and leave the host’s body. The Doctor and Donna separately infiltrate the offices of Adipose Industries, each unaware that the other is there. As they explore the building, they suddenly encounter each other through opposite windows in an office. They are confronted by Miss Foster, an alien who is using Britain’s overweight population to create the Adipose babies for the Adiposian First Family. Miss Foster pursues the Doctor and Donna around the building, finally catching them in an office. She tells the Doctor that the Adipose lost their breeding planet and hired Miss Foster to find a replacement. The Doctor uses Miss Foster’s sonic pen and his sonic screwdriver to create a diversion and escape.
Miss Foster accelerates her plans, knowing that the Doctor will attempt to stop her. Throughout London, the Adipose begin to spawn and soon number several thousand. The Doctor and Donna prevent total emergency parthenogenesis occurring, which would have killed those who had taken the pill, and the remainder of the young Adipose make their way to Adipose Industries. The Adiposian First Family arrive in a spaceship and begin collecting their young. The Doctor tries to warn Miss Foster about her safety, but she disregards him and is killed when the Adipose drop her from their transport beam to her death, to cover their unsanctioned colonisation efforts. The Doctor refrains from killing the young Adipose because they are children, to which Donna remarks that his previous companion Martha made him more human.
Donna accepts the Doctor’s original offer to travel in the TARDIS. Donna makes a detour to leave her car keys in a litter bin, telling her mother Sylvia to collect them later. While there, she meets a blonde woman and asks her to help Sylvia find the keys. The woman turns out to be Rose Tyler, who fades from view as she walks away from the area.