Partners In Crime

The episode primarily focuses on Donna Noble, a previous companion who appeared in "The Runaway Bride". After her encounter with the Doctor, she became disenchanted with normal life and regretted declining his invitation to travel in the TARDIS. She started investigating conspiracy theories in the hope she would find him. She confides her regrets to her grandfather Wilfred Mott, an amateur astronomer who met the Doctor before in "Voyage of the Damned".
The episode concerns Adipose Industries, which is marketing a diet pill to London's population with the slogan "the fat just walks away". Believing the treatment to be otherworldly, the Doctor and Donna investigate the company separately, and find that the slogan is literal—the pills use latent body fat to parthenogenetically create the Adipose, small white aliens which spawn every night, removing a little of the host's body fat each time. In an emergency, multiple Adipose can spawn by using all of the body's organic tissue, killing the host. When the Doctor and Donna meet, they are confronted by Miss Foster, an alien who is exploiting Britain's overweight population to create the Adipose for the Adiposian First Family. Miss Foster mentions that the Adipose species "lost" their breeding planet and hired Foster to find a replacement; she chose Earth, knowing that it was illegal.
Foster accelerates her plans, feeling threatened by the Doctor's invocation of galactic law and fearing he may inform the "Shadow Proclamation", an interplanetary police force. Throughout London, the Adipose begin to spawn, soon numbering several thousand. The Doctor and Donna prevent total emergency parthenogenesis occurring, while the remainder make their way to Adipose Industries. The Adiposian First Family use their spaceship to collect the Adipose, but kill Foster to hide any evidence they used Earth illegally. The Doctor refrains from killing the Adipose because they are children; Donna notes that his previous companion Martha Jones made him more human, citing his infanticide of the Racnoss in their previous encounter.
At the end of the episode, Donna accepts an offer to travel in the TARDIS. She makes a detour to leave her car keys in a safe location for her mother Sylvia, and asks a blonde woman to help Sylvia find the keys. The woman turns towards the camera, revealing her to be Rose Tyler. She fades from view as she walks away from the area. In the final scene, Donna asks the Doctor to fly by her grandfather, Wilfred, who sees her and celebrates on his allotment.