The TARDIS is drawn to the planet Messaline, where a war between human colonists and the piscine Hath has been waging for generations. Martha is kidnapped by the Hath, while the Doctor and Donna discover that the humans breed by accelerated progenation: a single individual's DNA is recombined to produce an adult ready for battle. Subjected to this process, the Doctor abruptly comes face to face with his daughter. As he struggles to relate to the girl, whom Donna names Jenny, they discover a series of hidden tunnels beneath Messaline. Soon, the time travellers start to realise that there is more to the war than meets the eye.

Stephen Greenhorn had written The Lazarus Experiment for Doctor Who's twenty-ninth season, and executive producer Russell T Davies was eager to invite him back the following year. While developing his preliminary ideas for Season Thirty in early 2007, Davies read an interview that Greenhorn had given to Doctor Who Magazine, in which he discussed the unchanging nature of the Doctor as a character. Davies decided to assign Greenhorn a narrative which would challenge this very notion, by pairing the Doctor with a character who would be his genetic offspring. However, unlike Susan -- the Doctor's granddaughter who had accompanied him during Doctor Who's first ten serials in 1963 and 1964 -- this new character would initially be conditioned with attitudes very much at odds with the Doctor's. As such, the episode would complement the preceding story -- eventually The Sontaran Stratagem / The Poison Sky -- which would explore the sometimes uneasy relationship between the Doctor and the Unified Intelligence Taskforce (UNIT).

Greenhorn began working on the script -- eventually given the title The Doctor's Daughter -- after meeting with Davies on March 13th. In addition to the Time Lord's newly-created progeny, Davies indicated that former companion Martha Jones should appear. Martha would inadvertently rejoin the Doctor in The Sontaran Stratagem / The Poison Sky, but the events of The Doctor's Daughter would prompt her to decide against further travels in the TARDIS. Other elements -- including the underground war and the Hath's mode of communication via bubbling liquid -- were also suggested by Davies.

Jenny was meant to die at the end of the story, until Steven Moffat suggested that this denouement was too obvious

One of the first set pieces conceived for The Doctor's Daughter was Jenny's acrobatic tumble between the laser beams; it was inspired by the 2004 music video for Britney Spears' Toxic. Other aspects of the script evolved considerably during its development, with various plot points -- such as the war's seven-day span, Donna's investigation of the mysterious numbers, and the greenhouse venue in which the Source was located -- added to later drafts. The resolution originally saw General Cobb taken away for execution, but this was replaced by the Doctor's threat to shoot Cobb, and his decision against doing so. For a long time, Jenny was meant to die at the end of the story, until Steven Moffat -- who was writing Silence In The Library / Forest Of The Dead for later in the season -- suggested that this denouement was too obvious. Instead, an epilogue was developed in which Jenny revived and headed off to new adventures.

The Hath initially fought humanoids called the Takrans, but the pronunciation of the name was found to be ambiguous, and Davies eventually decided that it was simpler if the Hath's opponents were simply humans. Even though they communicated by gurgling, Greenhorn wrote all of the dialogue for the Hath so that the actors under the masks would have an impression of how their characters should behave. The two main Hath were named Peck and Gable, after Hollywood icons Gregory Peck (To Kill A Mockingbird) and Clark Gable (Gone With The Wind).

The Doctor's Daughter was paired with Midnight as the sixth block of the Season Thirty production schedule. The director would be Alice Troughton, who had previously worked on Small Worlds and Out Of Time for the first year of Torchwood, plus Revenge Of The Slitheen and Eye Of The Gorgon for the inaugural season of The Sarah Jane Adventures. Troughton would initially focus on Midnight, since it was being double-banked with Block Seven, which consisted solely of Turn Left.

The critical role of Jenny went to Georgia Moffett who, ironically, really was the Doctor's daughter. Her father was Peter Moffett; under his professional name of Peter Davison, he had played the Fifth Doctor. Indeed, Davison himself was given the privilege of informing Moffett that she was to appear in Doctor Who, having learned of the casting decision while making the charity mini-episode Time Crash on October 7th. Moffett's involvement in The Doctor's Daughter was not a stunt: she was an experienced actress in her own right, with credits on programmes such as Peak Practice, The Bill and Where The Heart Is. She had also appeared alongside her father in the 2000 Doctor Who audio play Red Dawn from Big Finish Productions. Despite her heritage, Moffett had not been particularly interested in Doctor Who until her young son, Ty, became an avid devotee with the series' 2005 revival. Now a Doctor Who fan herself, Moffett had attracted the attention of the production team some months earlier, when she auditioned for the role of Robina Redmond in The Unicorn And The Wasp.

On December 11th, executive producer Julie Gardner's forthcoming departure was announced

Work on The Doctor's Daughter began on December 11th, the same day that Troughton taped the last material for Midnight. The venue was Doctor Who's regular studio facilities in Upper Boat, where the TARDIS material and the closing sequence of Jenny leaving Messaline were recorded. This was also the day that the BBC announced the forthcoming departure of Julie Gardner, who was stepping down as both the executive producer of Doctor Who and the Head of Drama for BBC Wales as of January 2009. She would be replaced in both roles by Piers Wenger, although she would maintain her involvement with Doctor Who until the last of the specials planned for 2009. Wenger had formally accepted the job on December 7th.

On December 12th, the scenes of Martha and Hath Peck on the surface of Messaline were filmed at the open cast colliery in Cefn Cribwr. All of the action at the Hath encampment was then scheduled to be recorded on the 13th, in the Marble Room at Cardiff City Hall. Unfortunately, the location was being picketed due to a controversy over school closures, causing Troughton's team to lose several hours, and forcing the postponement of a number of shots. The next location was the Celynen Colliers Institute and Memorial Hall in Newbridge, which offered space suitable for the human encampment. Work there began on December 14th and continued after the weekend, from the 17th to the 19th. Part of the 18th was also spent on the Hath encampment material which Troughton had been unable to record at Cardiff City Hall. The latter part of December 19th and all of the 20th took the cast and crew to the Dupont Building in Pontypool's Mamhilad Industrial Park, for sequences in the spaceship corridor. The final day of work on Doctor Who before the Christmas break was December 21st, when the greenhouse containing the Source was actually Plantasia in Swansea.

2008 was a week old by the time the cameras again rolled on The Doctor's Daughter. On January 7th, the tunnel where the Doctor located the secret panel was part of the Lewis Merthyr Colliery, in the Rhondda Heritage Park at Trehafod. Troughton's team then returned to the Mamhilad Industrial Park on the 8th and 9th, albeit now at the Johnsey Estates, for the material in the tunnel where Jenny disabled the laser beams. The sequences in the area where the TARDIS materialised were filmed on January 10th and 11th at the Barry Shooting Range, a disused railway tunnel in Barry. The last major scene on Troughton's agenda was Martha's return home, which was taped at Mark Street in Cardiff on January 18th. The same day, greenhouse inserts were captured at Roath Conservatory in Cardiff. Lastly, a pick-up shot of the toy mouse exploding was filmed at Upper Boat on the 24th.

During the early part of Season Thirty, Davies was vocally dissatisfied with the early timeslot accorded to Doctor Who. The Doctor's Daughter finally saw a later start, at 6.45pm, by virtue of Outtake TV's addition to the schedule earlier in the evening. Meanwhile, it transpired that the production of The Doctor's Daughter had seen an attraction develop between Moffett and David Tennant. The series star had recently concluded a two-year romance with Sophia Myles, who had played Madame de Pompadour in The Girl In The Fireplace, and he had formed a good relationship with Moffett's son, Ty, during his visits to the set. The pair began dating soon after finishing work on The Doctor's Daughter; they would marry on December 30th, 2011, with Tennant becoming Ty's adoptive father.

• Doctor Who Magazine Special Edition #20, 19th November 2008, “Episode 6: The Doctor's Daughter” by Andrew Pixley, Panini Publishing Ltd.
• Doctor Who: The Complete History #58, 2018, “Story 193: The Doctor's Daughter”, edited by Mark Wright, Hachette Partworks Ltd.
• Doctor Who: The Writer's Tale by Russell T Davies and Benjamin Cook (2008), BBC Books, ISBN 978 1 846 07571 1.