"The Unicorn and the Wasp" is the seventh episode of the fourth series of the revived British science fiction television series Doctor Who, which was aired on BBC One on 17 May 2008.
Set in an English manor house in 1926, shortly before the disappearance of crime fiction novelist Agatha Christie (played in the episode by Fenella Woolgar), the episode is a murder-mystery storyline where a shapeshifting giant wasp, in disguise as one of the party guests, murders the other guests using methods similar to those in the novels of Christie, who is also a guest.
Plot
The Tenth Doctor and Donna invite themselves to a dinner party hosted by Lady Eddison in 1926. One of the guests is Agatha Christie, and the Doctor realises that they have arrived on the day she inexplicably disappears. One of the guests, Professor Peach, is killed with a lead pipe in the library, where part of a burnt birth certificate is found.
While searching in a room that Lady Eddison shut herself in for six months in 1886, Donna sees a giant wasp outside. It smashes through the window and attacks Donna before escaping. The wasp then crushes the housekeeper, Miss Chandrakala, with a gargoyle. The Doctor, Donna, and Agatha chase the alien but it returns to human form before they can catch it. In the study, the Doctor identifies the killer as an alien wasp called a Vespiform. He is then mysteriously poisoned with cyanide, but detoxifies due to his Time Lord physiology. During a failed attempt to reveal the wasp at dinner, Lady Eddison's necklace is stolen and her son Roger Curbishley is stabbed in the back with a bread knife.
The remaining guests are assembled in the sitting room. When it is revealed the jewel thief’s toolkit Agatha found in the shrubbery likely came from Robina Redmond's bathroom window, Agatha reveals Redmond to be the jewel thief the Unicorn. The Doctor deduces that Lady Eddison's shutting herself away in 1886 was due to her becoming impregnated by another Vespiform in India, who had given her the necklace, before he died in the floods of 1885; unbeknownst to her, it links her telepathically with their child. Agatha deduces that Chandrakala took the child to an orphanage in 1886, and Professor Peach discovered Lady Eddison's secret. The Doctor further reveals that the child, whom she gave up for adoption, is Reverend Golightly, the killer. Via the telepathic link, the Reverend became aware of his alien nature, and inheritance, and absorbed the details of an Agatha Christie murder mystery that his mother was reading at the time.
Enraged, Golightly transforms into the Vespiform and threatens the guests. Agatha grabs the necklace and lures him away while driving towards the nearby Silent Pool. When Donna catches up with Agatha, she grabs the necklace and throws it into the water, prompting the Vespiform to dive in after it and drown. Due to her own connection with the necklace, Agatha falls unconscious and suffers from amnesia. The Doctor quietly drops Agatha off in Harrogate ten days later.
Continuity
The Doctor produces items from a chest of items beginning with the letter 'C', including a Cyberman chest-plate from "The Age of Steel", the head of a Greco-Roman statue (possibly depicting Caecilius from "The Fires of Pompeii"), and the crystal ball in which the Carrionites are trapped from "The Shakespeare Code" (which he playfully shakes).
Early in the episode, the Doctor states his desire to meet Agatha Christie. This is a reference to "Last of the Time Lords". Donna remarks that meeting Agatha Christie during a murder mystery would be as preposterous as meeting "Charles Dickens surrounded by ghosts at Christmas", unknowingly describing the events of "The Unquiet Dead". When Donna talks to Agatha about her cheating husband, she recalls her own engagement to Lance in which she found out he was using her, briefly mentioning the Empress of Racnoss as "a giant spider". This is a reference to the 2006 Christmas episode "The Runaway Bride".
The Doctor has a flashback scene when unravelling motives with Agatha Christie. He is shown in medieval Belgium with a bow and quiver of arrows on his back. His voiceover explains that he was deep in the Ardennes, looking for Charlemagne, who had been "kidnapped by an insane computer". Christie interrupts before he can paint a full picture; however the events are fully explored on Doctor Who's BBC website in the short story "The Lonely Computer."