The Doctor (David Tennant) and Donna (Catherine Hicks) invite themselves to a dinner party hosted by Lady Clemency Eddison (Felicity Kendall) and Colonel Hugh Curbishly (Christopher Benjamin) in 1926. One of the guests is Agatha Christie (Fenella Woolgar) and they have arrived on the very day she will disappear for ten days. Of course, one of the guests is found dead in the library.
The Doctor and Donna find a viscous substance left behind by the killer that the Doctor discovers is morphic residue, which means the killer is an alien in human form. While searching in a long-empty room, Donna sees a gigantic wasp outside. It smashes through the window and attacks Donna, embedding its sting in the door before escaping. Then it kills the housekeeper Miss Chandrakala (Leena Dhingra). The Doctor, Donna, and Agatha Christie chase it, but it returns to human form before they can catch it. Back in the study, the Doctor identifies the mysterious substance as the venom from a Vespiform—a giant wasp. He is poisoned with cyanide, but his Time Lord physiology protects him. Lady Eddison’s necklace is stolen, and her son Roger (Adam Rayner) is stabbed.
The guests are assembled in the sitting room. Agatha exposes Lady Robina Redmond (Felicity Jones) as the thief, but not the murderer. The Doctor deduces that Lady Eddison had shut herself away for months, years ago, because she was impregnated by a Vespiform. As a human, it had given her the necklace, which links her telepathically to her child. Further, the child she gave up for adoption is Reverend Golightly (Tom Goodman Hill). Telepathically, he learned of his alien nature because he had absorbed the details of Christie’s The Murder of Roger Ackroyd, which his mother had been reading at the time. Got that?
Reverend Golightly changes into the Vespiform. Agatha lures him away from the guests by grabbing the necklace and driving off toward the Silent Pool. Donna catches up with Agatha, grabs the necklace and throws it into the pool. The wasp drowns trying to retrieve it. Agatha falls ill and suffers from amnesia. The Doctor drops her off near the Harrogate Hotel ten days later, then shows Donna that her novel Death in the Clouds is about wasps, indicating that she remembered something of the adventure.
The episode was written by Gareth Roberts, who wrote the Shakespeare Code, and it was quite popular with fans and critics. David Tennant’s father Alexander McDonald, visiting the set, appeared as a footman in a non-speaking role. Donna remarks that meeting Agatha Christie during a murder mystery would be as preposterous as meeting Charles Dickens surrounded by ghosts at Christmas. Agatha Christie’s grandson sat in on script-readings and rehearsals. The role of Lady Eddison was offered to Helen Mirren, but she turned it down.
The car is a 1919-1926 Morris Cowley “Bullnose,” the same car Agatha Christie was driving when she disappeared. Athelhampton House, where the episode was filmed, was also the setting of Seeds of Doom with Tom Baker. There are at least twenty references to Agatha Christie books in the episode. The weapons used are a lead pipe, a revolver, a knife, a candlestick, and rope, and characters are based on Professor Plum, Reverend Green, Mrs. Peacock, Miss Scarlet, Colonel Mustard, and Mrs. White—all from the game Clue.