4.10 - Midnight

From IMDB:
The Doctor and Donna are taking a bit of a break from their adventures and spending a bit of time on the planet Midnight. While Donna opts to spend a bit of time in the spa, the Doctor goes off on an all-day bus tour to the Sapphire Waterfall. Part way through the four hour journey to get there, the vehicle stops for no apparent reason. When they hear noise coming from outside the vehicle, panic begins to set in. When one of the passengers is taken over by the being, the passengers paranoia and fear know no bounds and the focus of their solution is to eliminate the Doctor.

4 out of 108 (3.7%) raters say this story requires a previous story.