The Doctor is alerted to a disturbance in the time stream when guerrillas from the 22nd century appear. Their goal is to assassinate Sir Reginald Styles, who is about to host an important international peace conference. The Doctor learns that Styles is destined to blow up the conference, instigating World War Three. As a result, Earth two hundred years in the future is dominated by one race: the Daleks, aided by their brutish footsoldiers, the Ogrons.

It had been more than half a decade since Louis Marks contributed Planet Of Giants for Doctor Who's first production block when, in 1970, he was approached to submit ideas to the programme once again. Marks developed a concept in which guerrillas travel back in time to the present day to avert a military dictatorship dominating Earth in the future. An amended version of this plotline followed in which not only did the rebels travel back through history, but the Doctor also went forward to their era; this addition convinced script editor Terrance Dicks that the idea was worth pursuing.

On January 22nd, 1971, Marks was commissioned to write the storyline under the title “The Ghost Hunters”, with a commission for the scripts following on April 1st. By this time, Marks had joined the BBC staff as a script editor on Trial, and consequently permission had to be secured from the Plays Department for Marks to work on Doctor Who. “The Ghost Hunters” was now planned to be the lead story of the programme's ninth season and identified as Serial KKK. In devising his characters, Marks found inspiration in the September 1970 hijacking of three airplanes by the Popular Front for the Liberation of Palestine and so gave his time-travelling guerrillas Middle Eastern-like names.

Meanwhile, an inquiry by BBC Managing Director Huw Wheldon lead Dicks and producer Barry Letts to consider the return of the Daleks to Doctor Who. The monsters had not headlined a story since The Evil Of The Daleks at the end of Season Four, and since taking over Doctor Who during Season Seven, Letts had been generally reluctant to resurrect old foes. Nonetheless, the production team decided to meet with Dalek creator Terry Nation. Nation indicated that, while commitments to The Persuaders! prevented him from writing a new Dalek story himself, he was amenable to another author being brought in, as long as he was given the power of approval on such scripts and an on-screen credit. Permission for the use of the Daleks formally came on April 22nd.

For the first Dalek story in half a decade, Letts and Dicks turned to Robert Sloman, who was commissioned on May 25th for a storyline called “The Daleks In London”. This six-part serial was envisaged as the finale for Season Nine. However, Letts and Dicks soon became concerned that the season did not have a leadoff hook to entice viewers, in the manner of the introduction of the Master the previous year or the debut of Jon Pertwee's Doctor the year before. It was therefore decided to abandon “The Daleks In London”; Sloman would devise a new storyline for the season's last story while the Daleks would be inserted into Marks' tale.

Around the start of June, the revised Serial KKK became “Years Of Doom” and, a month later, “The Time Warriors”. Letts and Dicks were delighted by the notion of time paradoxes, adding parallel sequences in which the Doctor and Jo at the beginning of the story meet themselves at the end of the story, told from both perspectives. The Blinovitch Limitation Effect was also created as a handwaving excuse why the guerrillas could not repeatedly go back to the same moment in time to try to kill Styles. By mid-July, the adventure was called “The Day Of The Daleks”, and the production team was already planning a second Dalek serial for Season Ten. A further title change to simply “Ghosts” occurred toward the end of the month before it reverted to Day Of The Daleks, although the definite article would occasionally reappear at the start of the title during subsequent development.

Styles' house was originally called Austerley House in Marks' scripts, becoming Alderley, Austerly, Auderley and finally Auderly House. A more significant change was the realisation of the Daleks' lackeys. Referred to for much of the story's gestation as simply “Monsters”, they were initially envisaged as dog-like humanoids who spoke fluent English. It was director Paul Bernard who suggested that they should be slow-speaking monstrous apes, and the race's name became Ogorons and then Ogrons. Bernard had been a designer on shows such as The Avengers before becoming a director and producer, with credits including Z Cars and This Is... Tom Jones.

While Nicholas Courtney, John Levene and Richard Franklin had all been contracted for the entirety of Season Eight, it was decided to hire them only on a serial-by-serial basis for the new year. Franklin was the first to be contracted for Day Of The Daleks, on August 13th; Courtney's services were secured on the 26th and Levene's on September 1st. This approach was similar to that taken with Roger Delgado, who had also been a series regular for Season Eight but would now only be contracted to play the Master for specific stories. Jon Pertwee and Katy Manning, however, continued to be contracted for the entire season, and as such their involvement was ensured well in advance of the other principal actors: Manning had been issued her contract for a minimum of twenty episodes on May 14th, while Pertwee's had come through on May 18th.

As production loomed, Bernard learned that only three complete Daleks were still retained by the BBC (along with the lower section of a fourth). These three were refurbished for Day Of The Daleks. Two were given a grey-and-black livery while the third was painted in gold and black; this made a change from Doctor Who's monochrome years when the superior Daleks had borne a predominantly black colour scheme while the regular Daleks were grey and blue. No new Dalek casings were constructed for Day Of The Daleks, however, and -- together with the unique appearance of the Gold Dalek -- this meant that Bernard was faced with considerable limitations in planning the shots of the Dalek attack on Auderly House.

Production on Serial KKK began with an experimental session at BBC Television Centre Studio 4 on September 7th. The aim of this day was to test alternative key colours for the Colour Separation Overlay process, with yellow, green and purple tried in place of the traditional blue. Location filming then started on September 13th. The original venue chosen for Auderly House was Osterley Park House in Osterley, Middlesex, but this was changed at short notice to Dropmore Park in Burnham, Buckinghamshire. Cast and crew remained at Dropmore Park for the morning of the 14th. They then travelled to Harvey House at Brentford, Middlesex where the parking lot scenes were filmed, and finally to Bull's Bridge in nearby Hayes, which was the entrance to the rebels' tunnel. The vicinity also served as the futuristic wasteland. Filming continued at Bull's Bridge on the 15th and 16th.

The first studio block took place in TC4; episode one scenes were taped on Monday, October 4th, followed by episode two material on Tuesday the 5th. The second block then occurred exactly two weeks later, this time in TC8. Bernard again devoted each day to one episode. Jean McFarlane, who played Miss Paget, fell ill prior to recording and so her dialogue was given to Styles' aide, played by extra Desmond Verini. Another change to part four was the loss of dialogue establishing that all the Daleks infected with the Human Factor at the climax of The Evil Of The Daleks had been eradicated, indicating that that story was not the “final end” of the Daleks after all.

One notable aspect of part four was the inclusion of photocaptions of the First and Second Doctors -- the first visual reference to Troughton's Doctor since his final serial, The War Games, and to Hartnell's incarnation since The Power Of The Daleks, Troughton's debut adventure. To represent the First Doctor, two images from 100,000 BC and one from The Daleks were selected, while pictures from The Faceless Ones and The Invasion depicted the Second Doctor.

In editing, episode four was found to be overlong. Bernard had never been happy with the sequel to the part one sequence in which the Doctor and Jo meet themselves; this was intended to be the story's last scene, and Bernard thought it ended Day Of The Daleks on an anticlimactic note. Although Dicks championed its inclusion, the timing problems gave Bernard motivation to cut it.

Day Of The Daleks then kicked off Doctor Who's ninth season when part one was transmitted on New Year's Day 1972. The next week, episode two garnered an audience of 10.3 million, the first time the series had exceeded ten million viewers since part three of The Daleks' Master Plan more than six years earlier.

• Doctor Who: The Handbook: The Third Doctor by David J Howe and Stephen James Walker (1996), Virgin Publishing, ISBN 0 426 20486 7.
• Doctor Who: The Seventies by David J Howe, Mark Stammers and Stephen James Walker (1994), Virgin Publishing, ISBN 1 85227 444 1.
• Doctor Who Magazine #301, 7th March 2001, “Archive: Day Of The Daleks” by Andrew Pixley, Panini Publishing Ltd.
• Doctor Who Magazine Special Edition #2, 5th September 2002, “Family Affair” by Andrew Pixley, Panini Publishing Ltd.