4.1 - Partners in Crime

From IMDB:
Independently of one another, the Doctor and Donna Noble are investigating Adipose Industries who claim to have created a revolutionary diet pill. The head of Adipose, Miss Foster, runs a pretty tight ship and they have been quite successful. Donna visits several people who have taken the pills and as advertised, the pills seem to be working. Except of course for the person that virtually vanishes, excerpt for a strange little creature in the bathroom sink. The Doctor and Donna join up eventually and come to the realization that there is definitely something alien behind what is going on.

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