S04E07: 'The Unicorn and The Wasp'

The Doctor and Donna arrive in 1920s England and attend a function at a remote country house owned by Lady Eddison. Celebrated novelist Agatha Christie turns up and helps the time travellers investigate the death of Professor Peach, with The Doctor posing as Inspector John Smith.

The Doctor figures out that Peach was killed by a Vespiform, a huge wasp-like creature, and the trio are soon chased by the alien around the house. More deaths occur, including the housekeeper, but the attention soon shifts to the presence of The Unicorn - a mysterious jewel thief after one of Lady Eddison's possessions.

Agatha and Donna help The Doctor to survive sipping a drink laced with cyanide, as he uses substances including anchovies to neutralise the threat.

The Doctor gathers all the suspects in the study and exposes young lady Robina as the Unicorn and inadvertently outs Eddison's husband Colonel Hugh as a man who fakes his disability to keep his wife. However, Reverend Golightly is unmasked as the killer, as he is the long lost son of Lady Eddison, who had him after an encounter with an alien.

The necklace, given to Eddison by Golightly's father, keeps the Vespiform alive and supplies information. Hence the murders following the pattern of an Agatha Christie novel, as Golightly had absorbed the information from the books read by his mother.

Golightly turns into the wasp and pursues Agatha, who leads him to the river banks. Donna throws the jewel into the water and the creature flies in after it and dies. The event triggers several days of amnesia for the author.

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