The Tenth Doctor and Donna invite themselves to a dinner party hosted by Lady Clemency Eddison and her husband Colonel Hugh Curbishley in 1926. 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 suddenly found dead in the library.
The Doctor and Donna 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. While searching in a room that had previously been empty for 40 years, Donna sees a gigantic wasp outside. It smashes through the window and attacks Donna, embedding its sting in the door before escaping. The creature then kills the housekeeper, Miss Chandrakala. The Doctor, Donna, and Agatha chase the alien but it returns to human form before they catch it. When they regroup in the study, the Doctor identifies the substance from the sting as venom from an alien wasp called a Vespiform, before, all of a sudden, he is poisoned with cyanide. Luckily, his Time Lord physiology allows him to detoxify. During a failed attempt to reveal the wasp at dinner, Lady Eddison’s necklace is stolen, and her son, Roger, is stabbed in the back with a knife.
The remaining guests are assembled in the sitting room. Agatha exposes one of the guests, Lady Robina Redmond, as the thief but not the murderer when she says “toilet” instead of the upper class saying “loo”. Then the Doctor deduces that Lady Eddison’s shutting herself away for months, years ago was due to her becoming impregnated by another Vespiform, who had given her the necklace before he died in 1885; unbeknownst to her, it links her telepathically with their child. The Doctor further reveals that the child, whom she gave up for adoption, is Reverend Golightly. Via the telepathic link, the Reverend became aware of his alien nature and absorbed the details of The Murder of Roger Ackroyd, an Agatha Christie murder mystery his mother was reading at the time.
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 wasp 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 near the Harrogate Hotel ten days later, before showing Donna his copy of Death in the Clouds, in which wasps play a vital part, stating that Christie did remember, after all.