The Doctor and Donna arrive at the English country home of Lady Eddison and Colonel Curbishley. It is 1926, and rumours abound 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 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.

During Russell T Davies' first three seasons as the executive producer of Doctor Who, he developed an annual tradition of pairing the Doctor with a well-known historical figure. This had begun with writer Charles Dickens in 2005's The Unquiet Dead, and Davies wanted the Doctor to meet another famous author for Season Thirty's “celebrity historical”. The idea came from a 2006 conversation with producer Phil Collinson, who had mentioned his longstanding desire for an adventure in which the Doctor would encounter Agatha Christie -- the so-called Queen of Crime. Over the course of her extensive career, Christie published about eighty mystery novels and dozens of short stories, 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 1920's The Mysterious Affair At Styles, 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, Christie was honoured as a Dame Commander of the Order of the British Empire; she died in January 1976.

Davies saw enormous potential in Collinson's suggestion. Whereas previous “celebrity historicals” had aired early in the year, he positioned the Agatha Christie story as the seventh episode of the 2008 season, where it could provide a mid-year ratings boost. The writer he had in mind for the project was Gareth Roberts, who had contributed The Shakespeare Code for Season Twenty-Nine and was a fan of Christie's work. Following an initial meeting on March 13th, 2007, Davies and Roberts envisaged the narrative as being set in the mid-Sixties and featuring 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.

In December 1926, Agatha Christie disappeared without explanation for 11 days

Instead, it was decided to shift the events back to the mid-Twenties. This would not only give Roberts the chance to reflect many staples of Christie's writing, but also incorporate a piece of real history. In December 1926, in the midst of a profoundly unhappy period, the author had disappeared without explanation for eleven days. Her mother had passed away earlier that year, and Christie had newly learned that her husband was having an affair. She 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 incident provided an excellent hook for a Doctor Who adventure. Another story destined for Season Thirty, Planet Of The Ood, was already intended to have a wintry setting, so Roberts would fudge the actual history by placing the events of his script in the summer. Davies encouraged the injection of 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 he 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 no suitable creature could be derived from Christie's works. Roberts, however, had a childhood memory of coming across a cover illustration for a reprint of Christie's 1935 novel Death In The Clouds. Painted by Tom Adams and in use from about 1969, it depicted a biplane being menaced by what -- in perspective -- appeared to be a giant wasp. Roberts developed this notion into the Vespiform, prompting the title “The Wasp And The Unicorn”. When it was observed that the resulting acronym coincided with vulgar slang for female genitalia, the nouns were reversed, and the script became known as 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, while events in foreign locales became increasingly prominent in Christie's works following her 1930 marriage to her second husband, archaeologist Max Mallowan. Roberts and Davies competed with each other to find opportunities in the dialogue where references to Christie book titles could be inserted -- perhaps most infamously the Doctor's pun about “murder at the vicar's rage”, in reference to the 1930 Miss Marple novel The Murder At The Vicarage.

Characters in The Unicorn And The Wasp corresponded to the suspects in the boardgame Cluedo

Roberts' other major influence was the murder-mystery boardgame Cluedo (first released in 1949, and renamed Clue in North America). Six of the characters in The Unicorn And The Wasp corresponded to the six original Cluedo suspects -- Colonel Mustard (Colonel Curbishley), Professor Plum (Professor Peach), Reverend Green (Reverend Golightly), femme fatale Miss Scarlett (Miss Redmond), ageing dowager Mrs Peacock (Lady Eddison), and housekeeper 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 Christie'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 the author.

The Unicorn And The Wasp was made alongside Planet Of The Ood as Block Two of the Season Thirty 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 the 8th and 9th, scenes set on the grounds of Eddison Hall were filmed, and the flashback sequence of the Doctor hunting Charlemagne was also completed on the first day. 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.

After the weekend, cast and crew reassembled at Tredegar House in Newport. From August 13th to 15th, it provided the library, Robina's bathroom, the upper landing, the dining room, the kitchen, and various hallways. The flashback of the younger Christie at a party was also recorded there on the 13th. Work at Llansannor Court then resumed from August 16th to 20th, omitting only Sunday the 19th. Recording covered scenes in the sitting room, the study, Reverend Golightly's bedroom, the stairs, and additional corridors, alongside more material out of doors. Part of the 18th was also spent at St Senwyr's, located on the grounds of Llansannor Court, which posed as Golightly's church.

The only set constructed for The Unicorn And The Wasp was the locked room; it was erected at Doctor Who's usual studio facilities in Upper Boat for filming on August 21st. Harper then turned his attention to Planet Of The Ood. It wasn't until September 6th that the sequence of the Doctor, Donna and Christie travelling to the Silent Pool was taped, with the trio driving along Penylan Road in Newport and arriving at Cefn Mably Lakes in Cardiff. September 7th was spent at Hensol Castle in Hensol. It served as the exterior of the Harrogate Hotel, and was the venue for the flashback involving the youthful Lady Eddison. Also filmed there was material involving an elderly Christie -- now played by Daphne Oxenford -- in a hospital room.

As originally conceived, The Unicorn And The Wasp was framed by sequences set just prior to Christie's death in January 1976. The episode opened upon a dark and stormy night, with Christie tormented by her fragmentary memories of the Doctor. There was also a coda, in which the Doctor and Donna visited the dying Christie. To demonstrate her enduring appeal, they showed the author a copy of Death In The Clouds hailing from the year Five Billion. In editing, however, it became clear that The Unicorn And The Wasp was badly overrunning. It was agreed that the 1976 scenes were ultimately superfluous, and they were dropped. As a result, a new concluding sequence in the TARDIS was needed. It was recorded at Upper Boat on November 16th, along with a pick-up shot of Donna in the locked room.

The Unicorn And The Wasp was broadcast on May 17th, 2008, which was also the day of the FA Cup Final between Cardiff City and Portsmouth. The football match pushed all of the BBC One schedule farther into the evening than usual, with Doctor Who running at 7.00pm -- the latest it had aired all year. There was then a one-week interruption to Season Thirty, in order to accommodate the 2008 edition of the Eurovision Song Contest. This was the second year in a row that Doctor Who had taken a short hiatus to make room for the international music competition.

• 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 Complete History #58, 2018, “Story 194: The Unicorn And The Wasp”, edited by Mark Wright, Hachette Partworks Ltd.
• Doctor Who: The Writer's Tale by Russell T Davies and Benjamin Cook (2008), BBC Books.