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.
Early writing career Edit
Agatha Christie's first novel was published in 1920. (AUDIO: The Chimes of Midnight) Over the course of the following six years, Agatha wrote a further five novels, including The Murder of Roger Ackroyd. (TV: The Unicorn and the Wasp)
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. Agatha aided the Doctor in finding the killer. Their joint investigation saw her come 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. (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".
Later life Edit
Returning to her home at Ashfield in Torquay, Agatha wrote a letter to Sir Godfrey Collins, of William Collins, Sons & Co Ltd. In the letter, she suggested "a murder at a vicarage" as a possible idea for a book, and a "Miss (Donna? Jane?) Marple" as a "new nemesis for [those] criminals and murderers". (PROSE: Agatha Christie's Letter to Sir Godfrey Collins)
Subconsciously inspired by Donna Noble, Agatha did indeed create Miss Marple. Over the course of her later career, Agatha wrote further novels including Death in the Clouds and Murder on the Orient Express, both subconsciously inspired by her adventure with the Tenth Doctor, Donna Noble and the Vespiform. She was also knighted, and became "Dame" Agatha Christie. (TV: The Unicorn and the Wasp)
Agatha's characters included Hercule Poirot and Miss Marple. Works by Agatha Christie included The Murder of Roger Ackroyd, Murder on the Orient Express and Death in the Clouds. (TV: The Unicorn and the Wasp)
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.