This is far, far too long. It's practically a plot summary of the episode.
Dame Agatha Christie was a famous English writer best known for her murder mysteries. Her prolific body of work was in print billions of years after her death, establishing her as the best-selling author of all time. She was also known for having disappeared in 1926, being found in a hotel under an assumed name and claiming no memory of the ten days during which she had been missing.
In 1926, Agatha learned her husband had begun an extramarital affair with a younger woman. This revelation broke her heart and, following an exchange of words with her husband, Agatha officially disappeared. In truth, she attended Lady Eddison's party as guest of honour, carrying on with her life as social attitudes demanded. There, she met the Tenth Doctor and Donna Noble, who she quickly grew to trust when another guest, Professor Peach, was found murdered in the library.
The Doctor, who claimed to be from Scotland Yard, asked for Agatha's help in finding the killer. She agreed, though made clear she would do so for the sake of justice, not for his enthusiasm. At first, they got nowhere as more people got murdered, though she soon came across the Unicorn's burglary tools hidden in a flowerbed; this, along with a speech from the Doctor and some kind words from Donna, restored her lost confidence in herself and her works, and together they started to make some progress. She also discovered the existence of alien life when she came face-to-face with a Vespiform, a giant wasp-like creature.
With the Doctor's help, Agatha deduced that the Unicorn was pretending to be Robina Redmond, though she wasn't the killer; the killer was actually Reverend Arnold Golightly, the human-Vespiform hybrid son of Lady Eddison. Enraged at being discovered, the Reverend changed to his Vespiform form and chased Agatha to a lake; when Donna threw the Firestone he was linked to into the lake, the Vespiform charged in after it and drowned, nearly taking Agatha with him as her mind was also linked to it, but he let her go at the last second.
The stress from the link made her lose her memories; the Doctor took her forward eleven days and left her outside a hotel in Harrogate, where she revived with no conscious memory of the events. Fragments of memory later emerged in her storytelling, including Donna's suggestions of her later books, as well as the Vespiform's wasp-like appearance, which inspired the cover of a book, Death in the Clouds. (TV: The Unicorn and the Wasp)
Police investigation and media interest Edit
Following the disappearance, Deputy Chief Constable Kenward became a key figure in charge of the investigation, with police and local residents searching for the author. Two days after residents found Christie's car by the Silent Pool, there were still no discernible leads. A scene of crime expert was brought in from Scotland Yard and local army units were issued to help with the operation. Christie's friends and family were interviewed, and the media asked for anybody with information to contact their local police. Several theories were offered by various parties, but none could explain the unfathomable truth. Detective Edward Ransgard called the case "a real mystery".
Apart from The Murder of Roger Ackroyd, Christie had written five other novels by 1926, but had not yet written Murder on the Orient Express nor her Miss Marple series. A subconscious memory of the Vespiform remained with Christie; a wasp played a role in one of her later novels, Death in the Clouds. (TV: The Unicorn and the Wasp) Centuries after its publication, a copy of Murder on the Orient Express found its way into the possession of Professor Sarah Lasky. (TV: Terror of the Vervoids)
Christie's works were still in print as late as the year 5,000,000,000, including Death in the Clouds.
The Doctor's adventures offered subconscious inspiration for Agatha Christie. Donna Noble accidentally inspired her novel Murder on the Orient Express and the character Miss Marple. The Vespiform and the Doctor also inspired Death in the Clouds. Donna herself once remarked that she should have made Christie sign a contract on the Miss Marple series. (TV: The Unicorn and the Wasp)
Other information Edit
The Eighth Doctor had a signed first edition printing of The Murder of Roger Ackroyd which was missing the last page, leaving him to believe he would never find out who did it. (AUDIO: Storm Warning) He also had a complete set of Miss Marples, which Christie had signed without being prompted. (PROSE: The Time Lord's Story) He also once claimed Christie had travelled with him as a companion. (AUDIO: Terror Firma)
Despite this, the Tenth Doctor later declared to Martha Jones he wanted to meet her. (TV: Last of the Time Lords) According to one account, the Doctor met Agatha for the first time in his tenth incarnation. (TV: The Unicorn and the Wasp) This meeting was later recalled by the Thirteenth Doctor. (TV: Kerblam!)
Trying to remember the name of the St Agnes Abbey, Clyde Langer could only remember that he thought it was some lady writer's name or her old lady detective. He didn't remember the name until Sarah Jane Smith guessed Agatha Christie. (TV: Eye of the Gorgon)
Behind the scenes Edit
Actor's view/preparation Edit
In her Q&A for Doctor Who Magazine, Fenella Woolgar said she read The Murder of Roger Ackroyd and Christie's autobiography to prepare for the role. When she won the part, she was also reportedly the first casting suggestion from David Tennant to be approved.
Woolgar also said she approved of the idea of Christie being a companion of the Doctor, and when Doctor Who Magazine remarked, "Who'd've thought Agatha Christie could be sexy, eh?", she informed/reminded them that Christie had had quite a few admirers for her looks when she was younger. This would make sense — Christie was 36 at the time of her disappearance, which meant she would be the same age meeting the Doctor and Donna, and lived to be 85.
Deleted scene Edit
In a scene included in the Series 4 DVD, late in her life and soon before her death, Christie, who is played by Daphne Oxenford as an elderly woman, begins to experience dreams and flashbacks of her adventure with the Doctor. Ultimately, the Doctor and Donna visit her (in what appears to be immediately before Silence in the Library), at which point her memories of the adventure began to return - and the Doctor reminds her of what happened. This scene does not present a continuity issue, as Donna is seen to be basically parroting the words the Doctor said to her regarding Agatha being the best-selling writer of all time in the original ending, allowing both to exist. The fact the Doctor has chosen to inform Agatha about the future edition of her books as well as reminding her about what happened suggest the meeting takes place soon before her death, which would place the scene in 1976.