The Doctor and Donna arrive at an English country manor in 1926 amidst rumours that a jewel thief nicknamed “the Unicorn” is at large. But these stories are overshadowed by a murder in the library, and the timely arrival of famed suspense novelist Agatha Christie -- during a period in history when the Doctor knows that she is supposed to have vanished without explanation for several days. As the body count starts to climb, Donna is menaced by what appears to be a giant wasp, and only the Queen of Crime can help the Doctor to unravel the mystery.

In 2006, Doctor Who producer Phil Collinson mentioned to executive producer Russell T Davies that he had always longed for an adventure in which the Doctor would encounter British writer Agatha Christie. Known as the “Queen of Crime”, Christie published about 80 mystery novels and 160 short stories over the course of her extensive career, and she was acknowledged as the best-selling author of all time by the Guinness Book Of World Records. Her most famous literary creations -- Belgian detective Hercule Poirot (who first appeared in The Mysterious Affair At Styles in 1920) and elderly sleuth Miss Jane Marple (introduced in the 1927 short story The Tuesday Night Club) -- had become two of the mystery genre's most beloved and enduring characters. In 1971, she was honoured as a Dame Commander of the Order of the British Empire.

Davies was delighted by Collinson's idea, and thought that it would be an ideal project for Gareth Roberts. Roberts had written The Shakespeare Code for Doctor Who's 2007 season, and was scripting several episodes of The Sarah Jane Adventures (latterly Whatever Happened To Sarah Jane?); Roberts was also a fan of Christie's work. Davies planned for the Agatha Christie tale to be the seventh story of the 2008 season, filling the now-traditional “celebrity historical” slot occupied in the past by The Shakespeare Code (with William Shakespeare), Tooth And Claw (Queen Victoria) and The Unquiet Dead (Charles Dickens). Initially, Davies and Roberts envisioned setting the story in the mid-Sixties, depicting an elderly Christie who could be portrayed in the manner of Miss Marple.

However, they soon came to realise that a relatively modern setting would not convey the desired flavour of a classic Christie novel. Instead, it was decided to set the script in the mid-Twenties; this would give Roberts a chance to incorporate many staples of Christie's writing. One particular event in Christie's life invited particular attention: her disappearance for eleven days in December 1926. This was a profoundly unhappy period for Christie. Her mother had passed away earlier that year, and Christie had newly learned that her husband was having an affair. Christie vanished from their home in Sunningdale, Berkshire, and her car was found abandoned at Newland's Corner in Surrey, near a pond called the Silent Pool. She eventually surfaced at a hotel in Harrogate, Yorkshire, where it appeared that she was suffering from amnesia, possibly brought about by a nervous breakdown. At the time, however, the popular opinion was that Christie had merely staged a publicity stunt to help her book sales. Whatever the case, Roberts and Davies agreed that this provided an excellent hook for a Doctor Who story. It was agreed, however, that Roberts should deviate from recorded history by setting his script in the summertime, rather than December. This was because another story destined for the 2008 season, Planet Of The Ood, was already intended to have a wintry setting.

Roberts was working on his script by March 2007. Davies encouraged him to inject a strong element of comedy into the proceedings, and the centrepiece of this effort was the Doctor's poisoning, itself inspired by events in Roberts' Ninth Doctor comic strip The Love Invasion, published in Doctor Who Magazine during 2005. For a time, Roberts attempted to arrange his storyline so that suspicion for the murders could even be cast upon Christie herself, but ultimately found this angle to be unworkable.

As with the previous “celebrity historicals”, it was decided that Roberts' adventure should feature some kind of monster, but Davies worried that -- unlike Shakespeare or Dickens -- no suitable creature presented itself as an appropriate antagonist for Christie. Roberts, however, remembered a cover illustration for Christie's Death In The Clouds (originally published in 1935) that he had come across as a child. Painted by Tom Adams and in use from about 1969, this depicted a biplane being menaced by what -- in perspective -- appeared to be a giant wasp. Roberts developed this notion into the Vespiform, and the title of the episode became The Unicorn And The Wasp.

Roberts borrowed various elements from Christie's novels; for instance, Lady Eddison's first name, Clemency, was an homage to Clemency Leonides from the 1949 novel Crooked House. Roberts and Davies also injected references to numerous Christie titles throughout the script. Roberts' other major influence was the murder-mystery boardgame Cluedo (originally released in 1949, and renamed Clue in North America). Six of the chief suspects in The Unicorn And The Wasp corresponded to the six characters of the boardgame -- Colonel Mustard (Colonel Curbishley), Professor Plum (Professor Peach), Reverend Green (Reverend Golightly), Miss Scarlett (Miss Redmond), Mrs Peacock (Lady Eddison), and Mrs White (Miss Chandrakala) -- while many of the game's weapons and locations were also represented in the script.

In early August, David Tennant asked Davies to rewrite the climactic scene at the Silent Pool. Originally, the Doctor rammed the Vespiform into the pond with his automobile in order to save Agatha's life; Tennant feared that this came uncomfortably close to painting the Doctor as a murderer. By this point, Christie's grandson, Mathew Prichard, had become involved with The Unicorn And The Wasp. Prichard, representing Christie's estate, attended the episode's readthrough and gave his enthusiastic approval to Doctor Who's portrayal of his grandmother.

The Unicorn And The Wasp was recorded alongside Planet Of The Ood as Block Two of the 2007-2008 production schedule, with Graeme Harper directing. The initial order of business was three days from August 8th to 10th at Llansannor Court, situated at Llansannor in the Vale of Glamorgan. The Doctor Who team was now joined by Catherine Tate as Donna Noble, making her first appearance in front of the cameras as a series regular. During August 8th and 9th, scenes set on the grounds of Eddison Hall were filmed, along with the flashback sequence of the Doctor hunting Charlemagne. Tennant's father, Sandy McDonald, was present on the second day, and was cast as one of the footmen. On the 10th, recording moved inside Llansannor Court for the material in the drawing room.

From August 13th to 15th, the venue was Tredegar House in Newport. This provided the dining room, kitchen, library, Robina's bathroom, and various hallways. Also recorded there was the flashback of the younger Christie. Work at Llansannor Court then resumed from August 16th to 20th (omitting only the 19th). Recording covered scenes in the sitting room, the study, Reverend Golightly's bedroom, corridor sequences, and more material outdoors. Golightly's church was actually St Senwyr's, located on the premises of Llansannor Court.

The only set constructed at Upper Boat Studios for The Unicorn And The Wasp was the locked room; taping there took place on August 21st. Some time later, on September 6th, the sequence of the Doctor, Donna and Christie driving towards the Silent Pool was filmed on Pen Y Lan Road in Newport. The pond itself was actually Cefn Mably Lakes in Cefn Mably. The next day was spent at Hensol Castle in Hensol; this served as the exterior of the Harrogate Hotel, and was the venue for Lady Eddison's flashback to her youth as well as material involving an elderly Christie -- now played by Daphne Oxenford -- in a hospital room in January 1976.

The 1976 scenes included a pre-credits sequence in which Christie calls out to the Doctor upon a dark and stormy night, as well as a coda in which the Doctor visits the dying Christie to give her the copy of Death In The Clouds from the year Five Billion. In editing, however, it became clear that The Unicorn And The Wasp was badly overrunning its timeslot. Harper decided to cut both of the scenes set in 1976, and as a result a new concluding sequence in the TARDIS was needed. This was recorded at Upper Boat on November 16th, along with a pickup shot of Donna in the locked room.

• Outpost Gallifrey: New Doctor Who Series News Page, edited by Shaun Lyon.
• Doctor Who Magazine Special Edition #20, 19th November 2008, “Episode 7: The Unicorn And The Wasp” by Andrew Pixley, Panini Publishing Ltd.
• Doctor Who: The Writer's Tale by Russell T Davies and Benjamin Cook (2008), BBC Books, ISBN 978 1 846 07571 1.