4.9 - Forest of the Dead

From IMDB:
Donna finds herself in a world where she is happily married and has two children. She has virtually no memory of what may have happened to her in the Library or anything prior to that in the real world. When she does get a glimpse in her mind of her past life, it is quickly shut down. In the Library itself, the Doctor and Professor River Song try to find out what happened not only to Donna but to the over 4000 people the computer says were saved when the library was shut down 100 years ago. The solution to it all lies in understanding what the computer has been telling them all along.

109 out of 121 (90.1%) raters say this story requires a previous story.