The episode sees the Doctor (David Tennant) and Donna Noble (Catherine Tate) arrive at a dinner party hosted by Lady Eddison (Felicity Kendal) and her husband, Colonel Hugh (Christopher Benjamin). One of the guests is none other than Agatha Christie (Fenella Woolgar). Looking at a newspaper, the Doctor finds that it is the day of Agatha Christie’s famous unexplained disappearance. Just as this revelation is made, another guest, Professor Peach (Ian Barritt), is found by Eddison’s friend and companion Miss Chandrakala (Leena Dhingra) in the library, murdered with a lead pipe; Donna alludes to the similarity to the boardgame Cluedo. The Doctor finds morphic residue on the floor while examining the scene, meaning that one of the guests isn’t human.
The Gallifrey Guardian 