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 (December 8, 1926). 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.
Aided by Agatha, the Doctor interviews the guests while Donna goes looking for clues. She investigates a locked room, which the butler explains Lady Eddison had sequestered herself in while recovering from a bout of malaria contracted in India forty years earlier and they had left locked after her recovery. Donna is attacked by a giant wasp after tracing a buzzing sound to a window. She scares it off with a magnifying glass. It escapes and apparently retakes human form before they can catch up, killing Miss Chandrakala along the way. Her last words are "The poor little child." At this point it becomes clear that the murder is being played out like one of Agatha's novels.
While the three mull over the evidence they've gathered thus far, the Doctor is poisoned with cyanide; however, it is not as fatal for him as it is for humans, and an odd combination of ingredients with a shock (in the form of a kiss) from Donna allows him to detoxify himself. In return, the Doctor "poisons" the guests' dinner with pepper; naturally this is not harmful to humans, but it acts as an insecticide to wasps. A buzzing sound can be heard moments later, to which Lady Eddison exclaims, "It can't be!" The lights are blown out by a sudden wind and they again fail to ascertain the identity of the alien. Roger Curbishley (Adam Rayner), Lady Eddison's homosexual son, is murdered in the confusion, and Lady Eddison's necklace, 'The Firestone,' is stolen.
In the sitting room, the Doctor and Agatha reveal several secrets about the guests and hosts. Robina Redmond (Felicity Jones) is a thief called 'The Unicorn' who coveted the Firestone and stole it in the confusion. Colonel Hugh is not actually wheelchair bound as he appears to be; he faked the condition to make sure Lady Eddison did not leave him. The truth of Lady Eddison's bout of malaria is also revealed; she was actually made pregnant by an alien known as a Vespiform, who gave her the Firestone necklace. The necklace is psychically linked to her son, whom she had given up for adoption and never saw again. Her son is actually the Reverend Golightly (Tom Goodman-Hill), who had come to associate Agatha Christie's novels with the way the world must work because Lady Eddison had been reading one when his alien biology was awakened in a moment of anger, and had killed those who were working against him in the manner of one of her novels.
Golightly, now enraged once more at being discovered, transforms into his wasp form. Agatha snatches the Firestone, and Golightly pursues her since she is now linked to it. The Doctor and Donna follow after her. Agatha leads the creature to the Silent Pool, where Donna throws the necklace into the water. Golightly follows it in and thus drowns. Still linked to the necklace, Agatha nearly dies as well, but Golightly chooses to release her as his last act. The trauma causes amnesia, and the Doctor deposits her at the Harrogate Hotel ten days later, explaining her disappearance.
In the TARDIS, the Doctor produces one of Agatha's novels, Death in the Clouds, and points to the copyright page in the front. The publication date is listed as the year five billion; Agatha Christie is quite literally the most popular novelist of all time. The cover features a giant wasp, suggesting that the amnesia was not total (although the wasp in the novel is in fact of the normal variety--it only seems large against its background due to forced perspective).
Doctor David Tennant (Tenth Doctor)
Companion Catherine Tate (Donna Noble)
Fenella Woolgar – Agatha Christie
Felicity Kendal – Lady Clemency Eddison
Felicity Jones – Robina Redmond
Christopher Benjamin – Colonel Hugh
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)
Writer Gareth Roberts
Director Graeme Harper
Script editor Lindsey Alford
Producer Susie Liggat
Executive producer(s) Russell T Davies
Production code 4.7
Series Series 4
Length 45 minutes
Originally broadcast 17 May 2008