The Doctor and Donna arrive in England in the 1920s and find themselves at an English manor. There's a party going on and the Doctor quickly decides to attend. Meanwhile, inside the manor, Professor Peach has arrived and goes into the library where he goes through some papers. As he finds a secret that he's looking for, someone comes in. Peach recognizes the person but then says "It isn't possible!" as the person beats him to death with a lead pipe.
The Doctor and Donna bluff their way in and meet the manor's owners, Lady Eddison and her wheelchair-bound husband, Colonel Hugh. The other guests include the Eddisons' son Roger (who is flirting with the male servant Davenport), rarely seen socialite Robina Redmond, and local minister Reverend Golightly. They chat and Golightly mentions that he drove off two thieves who tried to break into the church the previous Thursday on his birthday, but he makes little of it. Lady Eddison mentions that she has a rare jewel, the Firestone, but they are being careful because a cat burglar, the Unicorn, may try and steal it.
The party's final guest arrives: mystery novelist Agatha Christie. Lady Eddison is a fan of the writer, having recently read one of her novels. The Doctor enthuses over Agatha's novels, catching her by surprise since she is still early in her career. As Donna chats with Agatha, the Doctor sees a newspaper and realizes the date is December 8, 1926. On that day in history, Agatha disappeared for several days and her car was found at the nearby lake, Silent Pool. She then later appeared at a hotel several miles away and claimed she had amnesia.
Lady Eddisons' companion Miss Chandrakala finds Peach's corpse in the library. Her screams draw everyone and the Doctor, Donna, and Agatha go to investigate. The Doctor and Agatha start to examine the crime scene and the Doctor notices her hide a piece of burnt paper from the fireplace. He then takes control of the situation, using his psychic papers to convince everyone he's an inspector from Scotland and Donna is his "plucky assistant." Donna is less than thrilled. After he ushers everyone out, he gives Donna a magnifying glass to check things out while he discovers a morphic residue on the floor. Agatha is skeptical of their investigation but the Doctor convinces her to help him. However, she says that she's doing it to solve the crime, not to help the Doctor who is overly enthused about the whole situation.
The Doctor talks to all of the party guests, and none of them have an alibi. Lady Eddison claims that she was alone in the upstairs Blue Room just prior to the party when she came down to meet the Doctor. Agatha finally shows the Doctor the piece of burnt paper, which has the word "maiden" on it. Donna goes up stairs and finds the Blue Room, but the door is lock. The Eddisons' butler, Greeves, lets her in and explains that it has been sealed for the last 40 years, since Lady Eddison recovered from malaria. After he leaves, Donna hears a buzzing noise and goes to the window, only to find a giant wasp outside. It smashes through the window and circles around to come at her, but she puts her back to the window and uses the magnifying glass to focus the sunlight on it. That distracts it long enough for her to get past it and out the door, although it slams its stinger through the wall as it closes. It flies out the window as the Doctor and Agatha come to check out the noise.
Chandrakala is in the kitchen preparing for supper and listening to the household staff gossip, and then realizes there's something she needs to tell Lady Eddison. She goes outside but then looks up to see a gargoyle statue tottering on the ledge. It falls on her, and the Doctor, Donna, and Agatha arrive. With her dying breath she gasps out "The poor little child." They spot the wasp and follow it upstairs but it stays ahead of them and flies down a hallway into the guest quarters and disappears. The Doctor realizes that the earlier morphic residue indicates it's a shape shifter, and it's transformed back into one of the guests to hide itself.
The Doctor gathers all the suspects in the study and turns the meeting over to Agatha. However, the mystery writer admits she hasn't solved anything and suspects that someone is deliberately duplicating her novels to commit the murders. Outside, she also admits to Donna that her husband isn't there because he's having an affair that she recently discovered. Agatha notices a leather case hidden in a flowerbed near the bathroom. They take it to the Doctor who identifies the items inside as thieves' tools. The Doctor has a cup of tea and tells Agatha and Donna that the stinger sample belongs to a Vespiform, an alien wasp-creature from a distant galaxy. Before he can say anything further, he goes into convulsions: the tea is poisoned! Concluding that cyanide was used, the Doctor runs for the kitchen and starts swallowing everything with protein in it in an attempt to stimulate his enzymes. Before he can expel it he tells them he needs a shock to his system, so Donna kisses him. The Doctor exhales the poison in a gaseous cloud, and tells Donna never to do that again.
That night, everyone sits down to dinner and the Doctor informs them that he has spiced the soup with pepper, which contains pepperdyne: a key ingredient in insecticides. The lights go out and a buzzing noise fills the room. Lady Eddison says "It can't be!" while everyone panics and the Doctor tries to get them out the door. The lights come back up and Roger is dead, stabbed in the back. Further, the Firestone has disappeared from around Lady Eddison's neck.
The Doctor and Agatha agree that the killer wants something and they must determine what. They call all the suspects together and Agatha announces her own deductions. She starts with Robina and notes that she had left the party briefly and used the bathroom underneath which the thieves' tools were found. "Robina" tossed them out the window to hide the incriminating evidence. "Robina" admits that she's the Unicorn and took the place of the real Robina to steal the Firestone. She gives them the Firestone back but notes that she's a thief, not a murderer.
Agatha then starts to talk to Colonel Hugh. He breaks down and stands up, revealing that he isn't really crippled. He admits to Lady Eddison that he faked his handicap so that she wouldn't leave him for another man. Agatha is taken aback, and says that she didn't suspect him: she was just going to tell him he wasn't the murderer.
Agatha then confronts Lady Eddison with the Firestone and asks her how she obtained it. She has already deduced that Lady Eddison acquired the Firestone in India at the same time she caught malaria… but it wasn't malaria. Agatha explains that Lady Eddison was pregnant, came back, and concealed the pregnancy and birth by claiming to have malaria and isolating herself in the Blue Room 40 years ago. Chandrakala came back with her from India and helped her conceal the birth.
The Doctor realizes there is more to the story and insists that Lady Eddison explain who the father was. She says that while in India she saw a glowing light in the sky. The next day she met Christopher and they fell in love. Finally he revealed that he was a Vespiform who had journeyed to Earth to take on human form and study the planet. However, he was killed during a monsoon and left her the Firestone. After the birth, Lady Eddison placed the baby in an orphanage and Professor Peach found out.
Agatha agrees that Lady Eddison is not the murderer, while the Doctor notes that the murderer is recreating Agatha's written novels. Based on her comments at the party, the Doctor realizes that Lady Eddison was reading an Agatha Christie novel the previous Thursday while she was wearing the Firestone. Thursday was the night that Reverend Golightly encountered the thieves at his church, the night of his birthday. The Doctor points out that a forty-year-old man would have trouble defeating two young thieves. When Golightly confronted the thieves, his anger set off a transformation into a Vespiform. That activated the Firestone, which contained the Vespiform racial memories and beamed them into Golightly's mind. However, since Lady Eddison was holding the Firestone at the time, it also transmitted her impressions of Agatha's novels, imprinting them on Golightly's mind as well.
Golightly starts to lose his temper again and begins to transform. Agatha grabs the Firestone and insists that since she helped create Golightly as a murderer, she needs to stop him. She gets in her car and drives to Silent Pool, as Golightly completes his transformation and flies after her. The Doctor and Donna take another car and follow them. Agatha arrives at the lake and throws the Firestone into the waters. Golightly flies after it and drowns, but Agatha starts to die as well through the mental link. As its dying act, the Vespiform breaks the link and saves her life. The mental link causes Agatha to go into shock and loses her memories, accounting for her mysterious disappearance. She leaves her car behind and the Doctor and Donna arrange for her to check into a hotel until she recovers her memory so the documented historical events can play out. Being British, Lady Eddison and the others are too polite to ever mention it again.
Back in the TARDIS, the Doctor assures Donna that Agatha will be remembered. Opening a chest marked "C," he shows Donna one of Agatha's novels, "Death in the Clouds." Agatha wrote it nine years after her disappearance, and it features a death by wasp sting. He notes that Agatha's books will live on long after her date and points out the publication date: the year five billion.