4.7 - The Unicorn and the Wasp

From IMDB:
The Doctor and Donna arrive at an English country estate in the 1920s and meet none other than the renowned murder mystery writer Agatha Christie. The Doctor also realizes the importance of the date - it's the day when Christie famously disappeared for 10 days creating headlines across the country and around the world. History records that she reappeared 10 days later claiming to have no memory of where she has been or what she has done. They no sooner arrive than there is a murder, which Donna can't help but find it amusing since the victim is Professor Peach who was killed in the library with a lead pipe. In true Agatha Christie's fashion, the solution to the murder and Agatha's disappearance is found in a false identity and events that occurred long ago.

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