The TARDIS materializes at an English manor ion the 1920s and the Doctor realizes a party is going on. Among the guest are Professor Peach and Reverend Golightly. The Doctor prepares to get himself invited, while Professor Peach goes into the library where he discovers a secret among the papers. Someone comes in and Peach recognizes them… just as they beat him to death with a lead pipe as he says it isn't possible.
The Doctor and Donna arrive and meet with their hosts, Lady Eddison and her husband Colonel Hugh. The Doctor bluffs his way in and Lady Eddison mentions the Unicorn, a jewel thief who has just struck and may try to steal something at the party. Reverend Golightly visits and they talk about how he beat off some thieves at the church. Roger, the Eddisons' son, secretly flirts with the male servant Davenport. The last guest to arrive is mystery writer Agatha Christie, and Donna and the Doctor are duly impressed. As they chat, the Doctor checks a newspaper and determines that it's December 8, 1926: the day that Christie mysteriously disappeared and her car was found at Silent Pool.
Lady Eddison's companion Miss Chandrakala discovers Peach's corpse while the Doctor tells Donna about how Agatha Christie disappeared for a day and claimed amnesia. When Chandrakala arrives, the Doctor, Donna, and Agatha go to investigate. The Doctor notices Agatha pick up a bit of burnt paper from the fireplace. The Doctor uses his psychic paper to identify himself as a Scotland Yard inspector and has everyone leave. He then finds morphic residue and provides Donna with a magnifying glass to investigate, while the Doctor suggests that Agatha work for him. She reluctantly agrees, unhappy at the Doctor's casual attitude toward the murder.
The Doctor then interrogates the suspects, starting with Reverend Golightly. He claims to be alone, while Roger claims to be alone while he was actually secretly meeting with Davenport. Robina Redmond, another guest, claims to have been alone while loading a gun for murder, and Colonel Hugh has trouble distinguishing between the modern day and his time in the Army looking at can-can girls. Lady Eddison says she was alone in her Blue Room until she met the Doctor.
The Doctor and Agatha consider the fact none of the suspects have an alibi, and Agatha reveals she picked up a piece of paper that has the word "maiden" on it. Meanwhile, Donna investigates a locked door and the older butler, Greeves, informs her that Lady Eddison recovered from malaria in it and it has been undisturbed in the last 40 years. He lets Donna in and she dismisses him, but as she investigates she hears a buzzing noise. She goes to the window and finds a giant wasp outside. It bursts into the room but she uses the magnifying glass to focus the sunlight and injure it so she can escape and close the door. The Doctor and Agatha come running but it leaves its stinger in the door and flies off.
Chandrakala is managing the house staff and realizes there is something she needs to ell Lady Eddison. She goes outside only to have a gargoyle statue fall on her. The Doctor and the others find her dying as she mutters, "The poor little child." They see the giant wasp outside and run upstairs to find it lurking on the upper floor. It attacks them and Donna drives it off with the magnifying glass and they follow… only to discover it has disappeared.
Everyone assembles in the study but Agatha admits she's only a writer and doesn't have a clue. Donna chats with her outside afterward and Agatha wonders if someone is mocking her by committing murders similar to her novels. Agatha admits she discovered her husband was having an affair and Donna admits she'll be famous for years. Agatha notes a case hidden in the nearby flowerbed. She takes it to the Doctor and they realize it contains thieves' tools. While sipping a cup of tea, the Doctor has analyzed a sample of the stinger and determines is a Vespiform from a distant galaxy. However, he goes into convulsions and they realize the tea has been poisoned with cyanide. The Doctor staggers into the kitchen and uses protein to stimulate his enzymes to overcome the poison. He finally says he needs a shock and Donna kisses him, which allows him to expel the poison.
That night at dinner, the Doctor realizes he's laced the soup with pepper, a primary ingredient which is used as an insecticide. The lights go out and they hear a buzzing noise as the Vespiform appears. As Lady Eddison says, "It can't be," everyone panics and when the lights come back up, Roger is dead and Lady Eddison's valuable necklace The Firestone has disappeared.
Afterward, Agatha concludes the killer wants something and the Doctor agrees, saying that she has lived and she knows about people. He then summons all of the suspects together and introduces Agatha, who starts with Robina. Agatha identifies her as an impersonator and explains how she tossed the thieves tools outside the window into the flower bed: she's the Unicorn. The Unicorn confesses and turns over the Firestone, but says she's a thief not a killer. Agatha then confronts Colonel Hugh, who gets up from his wheelchair. He's stayed in the chair to assure that his wife would stay with him out of sympathy. Agatha admits she had no idea he could walk and was just going to say he was innocent.
Agatha then picks up the Firestone and asks Lady Eddison to explain how she brought it back from India, then suffered from malaria and kept herself confined in her room. Agatha concludes that Lady Eddison came back from India pregnant and concealed it with the aid of Chandrakala. However, the Doctor realizes what is happening and has Lady Eddison explain: she saw a burning light in the sky during her stay in India and the next day met Christopher and they had an affair. Christopher then revealed he was a Vespiform who took human form to study Earth. He died during a monsoon but left her the Firestone. Lady Eddison then placed the baby in an orphanage but Professor Peach discovered the truth. Agatha notes that Lady Eddison is innocent of murder and turns it back over to the Doctor, who notes that Donna was right and the whole thing is being acted out as a murder mystery. The Doctor notes that Lady Eddison was reading a murder mystery the last Thursday, the same night that Reverend Golightly confronted the thieves in his church. The Doctor notes that he couldn't have defeated two strong men, and it has been forty years since Lady Eddison gave birth… and Golightly is celebrating his 40th birthday. When Golightly grew angry at the thieves, he transformed for the first time. The Firestone is a Vespiform memory device that beamed his true identity into his mind, as well as absorbing the works of Agatha Christie from Lady Eddison's memories.
Golightly starts to lose his temper and then reverts to his Vespiform. Agatha says that her imagination is responsible and she'll find a way to stop it. She drives away in her car and the Vespiform pursues her, while the Doctor and Donna take another car and continue the chase. She drives to the nearby lake, the Silent Pool, where her car was found during her disappearance. She uses the Firestone since she is linked to it through the Vespiform. She throws it into the lake and it flies in after it, drowning. Agatha starts to die as well, connected to the Vespiform through the Firestone. However, the Vespiform releases her from the mental link before dying. Christie suffers from amnesia and the Doctor realizes they've solved the mystery of her disappearance. Lady Eddison and the others will never speak of it.
The Doctor and Donna look on as Agatha goes to the hotel where she will eventually be discovered. He figures that all of the events will eventually bleed through and influence her writing, including "Death in the Clouds"… featuring wasps and still published in the year five billion.