The Unicorn and the Wasp

The Doctor and Donna Noble arrive in 1926 and are invited to attend a dinner party hosted by Lady Eddison and her husband, Colonel Hugh. They are thrilled to find one of the guests is Agatha Christie, and realize that today is when she will inexplicably disappear for ten days. As the party ensues, some of the guests are found dead, following patterns from Agatha's books, though the Doctor finds traces of alien morphic residue at one of the scenes. Donna also encounters a giant wasp, which she is able to scare off; as it flies off, it morphs into a humanoid shape and eludes her. The Doctor himself becomes poisoned with cyanide but his Time Lord physiology allows him to detoxify himself with a combination of ingredients and a "shock" provided by Donna kissing him. This inspires him to add pepper to the dinner meal; while harmless to humans, it would act as an insecticide due to piperine. As they eat, the guests hear the wasp sound, but the lights in the room are blown out before they discover the identity of the alien. When the lights are restored, they have found that Lady Eddison's Firestone necklace has been stolen, and that Roger, Lady Eddison's son, has been murdered.
The Doctor assembles the remaining guests in the sitting room, and first Christie, then the Doctor himself, reveal several truths they've recognized about the guests, including the identity of the thief who stole the Firestone. Most importantly, he has determined that Lady Eddison's claim of suffering from malaria years ago was due to her becoming impregnated by a Vespiform, who had also given her the Firestone necklace as a means of linking her telepathically with her child. Eddison ultimately left the boy for adoption. The Doctor reveals that child is the Reverend Golightly (Tom Goodman-Hill), and the murderer of the guests; his use of Agatha's works as means for murder was a result of his connection to his mother, who was reading one of Agatha's novels while the alien powers in Golightly manifested themselves for the first time in a bout of anger.Golightly reveals his alien nature, transforming back into a wasp and threatening the guests. Agatha grabs the Firestone necklace and lures the wasp away while driving towards the nearby Silent Pool, with the Doctor and Donna giving chase. When they catch up to Agatha, Donna grabs the necklace from Agatha and throws it into the water, the wasp diving in after it and drowning. However, due to her brief possession of the necklace, Agatha herself is lured to the water, but Golightly releases her from its possession before he dies, causing her to fall unconscious on land. The Doctor realizes this is the event that gave her the amnesia during her disappearance, and uses the TARDIS to quietly drop her near the Harrogate Hotel ten days later. As the two return to the TARDIS, the Doctor shows Donna that not only that Agatha's works will be remembered for a long time by producing one of her novels, Death in the Clouds, published in the year five billion, but that her amnesia may not have been complete, with wasps being a significant plot element of the work.