This episode marks the return of Donna Noble who becomes the new Doctor's assistant. Donna no longer works for the same company that first saw her on Doctor Who - The Runaway Bride. Donna seems to be doing some undercover work, trying to get to the bottom of things to do with Adipose technologies and their revolutionary slimming pill. The Doctor is also investigating but neither paths meet. Their paths eventually meet when they spot each other at the headquarters of Adipose Technologies, she looking through a window, he outside on a window cleaning barge. It transpires that Miss Foster, in charge of the company was using the pills to make new Adipose creatures from the fats of people.
Miss Foster believed if she helped the Adipose, they would reward her. When the little fat creatures came to life, they were picked up by a U.F.O. and taken away, Miss Foster is betrayed by the Adipose and they let her fall to the ground killing her when they turn off her tractor beam.
The Doctor agrees to let Donna Noble come with him to new and challenging adventures. It also marks the appearance of Wilfred Mott, the grandfather of Donna Noble who is an UFO fanatic but doesn't spot the Adipose U.F.O.
There is a brief appearance by Rose Tyler who is a face in the crowd but doesn't get involved. She disappears into the darkness after her appearance at the end.