Penny Carter was a journalist, working as science correspondent for The Observer. She attended a press conference at Adipose Industries. She was very sceptic about the diet pill, and decided to investigate it a bit further. She stayed at Adipose Industries till after closing time, hidden in a toilet stall. She was discovered by Miss Foster, thus saving Donna who was hiding in a stall near by.
Miss Foster took Penny to an office, and her security men tied her into a chair. She was later saved by the Doctor. (DW: Partners in Crime)
Behind the Scenes
Before it was widely known that Donna Noble would be returning during Series 4, the writers were told that the companion would be a woman named Penny who would have a similar personality to Donna. Penny Carter seems to be the result of that previous creation.