Doctor: David Tennant (Tenth Doctor)
Companion: Catherine Tate (Donna Noble)
Billie Piper – Rose Tyler
Sarah Lancashire – Ms. Foster
Bernard Cribbins – Wilfred Mott
Jacqueline King – Sylvia Noble
Verona Joseph – Penny Carter
Jessica Gunning – Stacey Campbell
Martin Ball – Roger Davey
Rachid Sabitri – Craig Staniland
Chandra Ruegg – Claire Pope
Sue Kelvin – Suzette Chambers
Jonathan Stratt – Taxi driver
Directed by James Strong
Written by Russell T Davies
Script editor Lindsey Alford
Produced by Phil Collinson
Executive producer(s) Russell T Davies
Incidental music composer Murray Gold
Production code 4.1
Series Series 4
Length 50 minutes
Originally broadcast 5 April 2008
Donna Noble has become disenchanted with her normal life for two years since meeting the Doctor in "The Runaway Bride". She finds herself regretting her decision to decline the Doctor's invitation to travel in the TARDIS. She has started investigating conspiracy theories in the hope that she will find him again. She confides her regrets to her grandfather Wilfred Mott, who met the Doctor before in "Voyage of the Damned".
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 pill's slogan, The Fat Just Walks Away, is literal. 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. Donna hides out in a toilet cubicle until they close, the Doctor stays in a storage room. 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.
The device that the Doctor and Donna use to stop Miss Foster's plans, as shown at the Doctor Who Experience.
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 colonization efforts. The Doctor refrains from killing the young Adipose because they are children, to which Donna remarks that his previous companion Martha Jones made him more human.
At the end of the episode, Donna accepts the Doctor's original offer to travel in the TARDIS. The Doctor makes it clear to her that he only wants to be friends and nothing more, and she emphatically agrees. She 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. In the final scene, Donna asks the Doctor to fly by her grandfather, Wilfred. He sees her in his telescope and celebrates on his allotment.