Doctor: David Tennant (Tenth Doctor)
Companion: Catherine Tate (Donna Noble)
Fenella Woolgar – Agatha Christie
Felicity Kendal – Clemency, Lady Eddison
Felicity Jones – Robina Redmond
Christopher Benjamin – Colonel Hugh Curbishley
Tom Goodman-Hill – Reverend Golightly
Ian Barritt – Professor Peach
David Quilter – Greeves
Adam Rayner – Roger Curbishley
Daniel King – Davenport
Charlotte Eaton – Mrs. Hart
Leena Dhingra – Miss Chandrakala
Alexander McDonald – Footman (uncredited)
Directed by Graeme Harper
Written by Gareth Roberts
Script editor Lindsey Alford
Produced by Susie Liggat
Executive producer(s) Russell T Davies
Incidental music composer Murray Gold
Production code 4.7
Series Series 4
Length 45 minutes
Originally broadcast 17 May 2008
The Doctor lands the TARDIS in England in 1926. He and Donna invite themselves to a dinner party hosted by Lady Eddison and her husband Colonel Curbishley. They are thrilled to find one of the guests is Agatha Christie, and the Doctor realises that they have arrived on the day that she will inexplicably disappear for ten days. One of the guests is found dead, and the Doctor uses his psychic paper to convince the rest of them that he is from Scotland Yard and Donna is his assistant.
Together with Agatha, he begins investigating. They discover a viscous substance left behind by the killer that the Doctor identifies as morphic residue. He concludes that the murderer is an alien in human form. The Doctor and Agatha question the guests while Donna searches the bedrooms for clues. Donna is attacked by a gigantic wasp, but it escapes through a window before the Doctor and Agatha arrive. The alien kills the housekeeper, and the Doctor, Donna, and Agatha chase it but it returns to human form before they catch it. When they regroup in the study, the Doctor is poisoned with cyanide. His Time Lord physiology allows him to detoxify, with Donna's help. The poisoning inspires him to add pepper to the dinner meal, since the piperine in it would act as an insecticide. As they eat, the guests hear the wasp but the lights in the room are blown out before the alien's identity is revealed. When the lights are restored, they discover that Lady Eddison's necklace has been stolen and that her son Roger has been stabbed to death with a knife.
The Doctor assembles the remaining guests in the sitting room, where he and Agatha reveal what they have discovered. Agatha exposes the Unicorn (the thief, but not the murderer). Then the Doctor deduces that Lady Eddison's shutting herself away for months, years ago, allegedly due to malaria, was actually due to her becoming impregnated by a Vespiform, an alien wasp who could transform into a human. The Vespiform gave Lady Eddison the necklace before he died; unbeknownst to her, it links her telepathically with their child. The Doctor further reveals that the child, whom she gave up for adoption, is really Reverend Golightly. Recently, during a bout of anger and via the telepathic link, the Reverend had become aware of his alien nature and absorbed the details of the murder mystery his mother was reading at the time, Christie's The Murder of Roger Ackroyd. The details of the book prompted him to kill in a fashion inspired by it.
Golightly transforms into a wasp and threatens the guests. Agatha grabs the necklace and lures him away while driving towards the nearby Silent Pool, with the Doctor and Donna close behind. When they catch up with Agatha, Donna grabs the necklace from her and throws it into the water, prompting the wasp to dive in after it and drown. Due to her own connection with the necklace, Agatha suffers as the wasp dies, but right before the end he severs the link, sparing Agatha but rendering her unconscious. The Doctor realises this is the event that gave her the amnesia during her disappearance, and uses the TARDIS to quietly drop her off near the Harrogate Hotel ten days later. Back in the TARDIS, the Doctor shows Donna Agatha's novel Death in the Clouds, in which wasps play a significant part. The Doctor explains that his copy was printed in the year 5,000,000,000 and that Agatha Christie is the best-selling novelist of all time.