4.3 - Planet of the Ood

From IMDB:
Donna is terrified at her first off-world trip where she and the Doctor encounter the Ood. It's the year 4126 and they seem to have arrived at factory where the Ood are prepared for sale to anyone willing to pay the price. The Ood from all appearances are a subservient race and are fitted with a communications device. Some of them however seem to be going rabid with their eyes turning a bright red. The company has been selling the Ood for some 200 years now and while it all seems benign on the surface, there is something far more sinister at work here.

15 out of 99 (15.2%) raters say this story requires a previous story.