E is for... Eddison, Lady

In 1926 Lady Clemency Eddison invited some friends to her country home to spend the weekend in the company of the celebrated crime writer Agatha Christie. As well as her husband, Colonel Hugh Curbishley, and her son Roger, the attendees included the Rev. Arnold Golightly, Professor Peach and Miss Robina Redmond. When the TARDIS materialised in the grounds, the Doctor and Donna gatecrashed the party. Prof. Peach was found murdered in the library, and the Doctor claimed to be a police detective, allowing him to investigate the crime with Mrs Christie. More killings occurred, and it was discovered that the culprit was an alien Vespiform - a huge wasp-like creature. Vespiforms could assume the appearance of other beings, so it became clear that one of the family members or their guests was an impostor. The Doctor soon discovered that Lady Eddison had returned from India some 40 years ago pregnant. Her housekeeper, Miss Chandrakala, had helped keep this secret. The child - a boy - was placed in an orphanage. Lady Eddison revealed that she had met a charming young man whilst in India, but he was really a disguised Vespiform. She had fallen pregnant by him but he later drowned in a flood. He had gifted her a fabulous jewel - the Firestone - which was really a Vespiform psychic device.
A huge fan of Mrs Christie's works, the Firestone had channeled her thoughts into her abandoned son just as his true identity asserted itself during a moment of great stress. This was the Rev. Golightly, who discovered his true nature when confronted by thieves in his church. His mind filled with details of Agatha Christie plots, he killed Professor Peach as he had worked out his real parentage, and Miss Chandrakala as she knew of the pregnancy. He then murdered Roger out of fraternal jealousy.
As well as losing both her sons, Lady Eddison also discovered that her husband was not an invalid as he claimed. He had been feigning illness as he was worried she might leave him.

Played by: Felicity Kendal. Appearances: The Unicorn and the Wasp (2008).