The concept of the Daleks and the Cybermen both appearing on-screen was first proposed in 1967, but was vetoed by Terry Nation, the creator of the Daleks. The episode is the first conflict between the two species in Doctor Who's 45-year history, and features Billie Piper's last appearance in the lead companion role as Rose Tyler; the final regular appearance of Noel Clarke as Rose's ex-boyfriend and previous companion Mickey Smith; and the final regular appearances of Camille Coduri and Shaun Dingwall as Rose's parents, Jackie and Pete Tyler. The episode was filmed in December 2005 and January 2006, alongside the episodes "Rise of the Cybermen" and "The Age of Steel".
The plot consists mostly of the Daleks and Cybermen waging a global war, with humanity caught in the crossfire. The Doctor, the Tyler family, and Mickey Smith fight for their lives trying to reverse the situation. They are successful, but at an emotional cost to the Doctor and Rose, as they are left in separate universes.
Continuing the events of "Army of Ghosts", the episode opens with Dr. Singh, Mickey, and Rose trapped in a sealed room within Torchwood. Four Daleks, accompanied by a device known as the "Genesis Ark", have emerged from the Void ship. A black Dalek called Dalek Sec extracts information about Earth from Dr. Singh, killing him in the process. Dalek Sec discovers that a separate invasion is in progress, and sends Dalek Thay to investigate. The Cybermen who took control of Torchwood detect the Dalek technology and confront them, offering an alliance. The Daleks decline and kill two Cybermen, causing the Cyber Leader to declare war on the Daleks.
Ms. Hartman and Jackie are taken by the Cybermen for conversion, but the Cyber Leader orders the Doctor held aside because he has valuable information about the Daleks. Ms. Hartman is converted, but before Jackie can be converted the Cyber Leader is destroyed by a strike team led by Jake Simmonds, who have travelled from their universe in pursuit of the Cybermen. Jackie escapes into a stairwell, and Jake uses a device around his neck to take the Doctor to the parallel universe and their version of Torchwood. The Doctor meets with the alternate universe Pete Tyler, who tells him that after he left the Cybermen were sealed in their factories while humanity decided what to do with them. In the meantime, the Cybermen vanished and the breach began causing unprecedented global warming on the parallel Earth, which the Doctor theorises is the start of the process that will lead to both planets falling into the Void. Pete demands the Doctor seal the breach before his Earth is destroyed.
Meanwhile, Rose deduces that she and Mickey were kept alive because their touch can activate the Genesis Ark. Dalek Sec explains that they cannot open the Ark because it is stolen Time Lord technology. He demands that Rose open it, but she refuses and mocks the Daleks until the Doctor arrives. The Doctor realises that these Daleks are the enigmatic Cult of Skaro, and uses his sonic screwdriver to allow the Cybermen into the sphere chamber. As the Cybermen attack the Daleks, Mickey accidentally activates the Ark while escaping. The Daleks kill the Cybermen and take the Ark outside to release its contents. The Ark opens and millions of Daleks that were imprisoned during the Time War pour out and begin engaging the Cybermen firing on them from below, though humanity is caught in the crossfire and many are killed while running for cover.
The Doctor and his companions flee into the tower. Along the way, the alternate universe Pete finds and saves Jackie. The Doctor brings everyone to the control room where he explains that crossing the Void causes a traveller to become saturated in Void material. If he opens the breach and reverses it, anything saturated in Void material will be pulled in. Everyone except Jackie has crossed the breach and is vulnerable to being trapped in the Void, so the Doctor sends them all to the parallel universe. Rose decides she would rather be with the Doctor than her family and jumps back to help him. The Cybermen attempt to stop the Doctor but are repelled by a converted Ms. Hartman, who has resisted the effects of her conversion and at least partially retains her personality. The Doctor and Rose open the breach and hold on to magnetic clamps as the Cybermen and Daleks are pulled in. The Cult of Skaro use an emergency temporal shift to escape. Rose's lever slips, and in resetting it she loses her grip and plunges toward the Void. At the last second, Pete reappears and grabs her, and together they transport back to the parallel universe. The breach closes, leaving a devastated Rose trapped in the parallel universe. The Doctor presses his ear against the wall, as a tearful Rose does the same in the other universe, as if trying to listen for each other.
Some time later, Rose has a dream where she hears the Doctor's voice calling her. The Tylers follow the voice to a remote bay in Norway called Bad Wolf Bay, where a holographic image of the Doctor appears. He tells Rose he is using the energy of a supernova to transmit to her via one last small breach between universes. Rose breaks down in tears and tells the Doctor that she loves him, but before the Doctor can reply, the breach seals and the Doctor's image disappears. In the TARDIS, tears run down the Doctor's cheeks as he slowly regains his composure and sets a course. As he gets back to piloting the TARDIS, he looks up to see a woman in a wedding dress standing in the TARDIS console room. With the Doctor shocked and confused, the bride angrily demands to know where she is, leading to the events of "The Runaway Bride".
The concept of the Daleks and Cybermen appearing together on screen is not new; in December 1967, the BBC approached Terry Nation to have both races in a serial, but Nation vetoed this idea. The concept came to Davies while mapping out the 2006 series: the story would both serve to resurrect the popular Daleks and provide a suitable exit for Piper, who had decided to leave Doctor Who. "Doomsday" is the first episode in the history of Doctor Who where the Cybermen and the Daleks appear on-screen together; Cybermen and Daleks were both featured in The Five Doctors and "Army of Ghosts", but in separate scenes.
The two-part finale was originally going to take place in Cardiff on the time rift, which was the focus of the episodes "The Unquiet Dead" and "Boom Town". When Torchwood was commissioned in 2005, Davies decided to base the spin-off in Cardiff and relocate "Army of Ghosts" and "Doomsday" to Canary Wharf in London.
An item of discussion between the production staff was over who would rescue Rose; Davies and Julie Gardner wanted Pete to rescue her, while Clarke and Phil Collinson wanted Mickey. The role was ultimately given to Pete, to emphasise that he had accepted Rose as a surrogate daughter. The Doctor's intended reply to Rose was also discussed; Davies, who left the reply unspecified, stated he didn't know when asked by Collinson on the episode's commentary track, and Gardner vehemently believed the Doctor would reciprocate Rose's love.
Some elements of the story were inspired by Philip Pullman's His Dark Materials trilogy. Pullman was "flattered" by the references in the episode, and compared Davies' actions to his own practice of referencing works.
Southerndown beach in Wales was used as the backdrop to the Doctor's farewell to Rose Tyler on Bad Wolf Bay.
To ensure that Clarke and Dingwall were available for filming, the story was filmed in the season's third production block with "Rise of the Cybermen" and "The Age of Steel". Filming for the story started on 2 November 2005 on location in Kennington, London, but did not become the primary focus of the production crew until 29 November, when filming began on the scenes in and around the sphere chamber. The scene of the Tylers driving through Norway was filmed at Bridgend on 6 December. Scenes in the lever room, the main setting for the story, were filmed on 12–15 December 2005 and 3–5 January 2006. Greenscreen work for Rose being sucked into the void took place on 13 January, and the skirmish between the military and Cybermen on the bridge was filmed on 15 January.
Other location shooting took place at the Coal Exchange and Mount Stuart Square, Cardiff Bay.
The penultimate scene of the episode, the Doctor's farewell to Rose, was filmed on 16 January 2006; it was the last day of filming for Clarke and Dingwall. As with all scenes set at Bad Wolf Bay, these were in fact filmed at Southerndown beach in the Vale of Glamorgan. Piper's last scene was Rose's reunion with the Doctor in "The Satan Pit" on 31 March, but the shoot was rather emotional, to the point there were several tears on set. The last scene of "Doomsday", Catherine Tate's appearance in the TARDIS as Donna Noble (credited as "The Bride"), was filmed on 31 March during the wrap party. To ensure the secrecy of Rose's departure and Tate's appearance, only Piper and Tennant were given scripts of the departure scene, and director Graeme Harper was not informed of the final scene until the last possible second.
As well as using existing music, such as the themes for the Daleks, Cybermen, and Rose, Murray Gold specially composed a piece of music for Rose's farewell entitled "Doomsday", which featured vocal work from Melanie Pappenheim. Instead of using the swelling violins that Davies and the rest of the production team had expected, Gold took a minimalist approach. When pitching the track to the production team, Gold described the track as representing Rose's unbridled energy and determination as she searches for the Doctor. He later said, "I wanted to get that kind of throbbing, sort of hurt sound of quite emotional rock, because I thought that's what Rose would do if she was hurting and ran up to her bedroom and locked herself in her room and had a good old cry, really." The piece uses the same vocal work from "Rose", when Rose first enters the TARDIS, thus creating a bookend effect. It is a favourite among fans and of executive producer Julie Gardner, and is one of the reasons, along with Pappenheim's overall contribution and the "Song for Ten" from "The Christmas Invasion", that the soundtrack of both series was released several months later.
To protect as much information concerning the episode as possible, the final scene of "Army of Ghosts" was withheld. The BBC website's Fear Forecasters, a panel who rate the episodes, were not allowed to see "Doomsday" before its airing, and access to copies was restricted; the website thus does not have a Fear Forecast for the episode. Despite this, the Dalek Sec prop, which had been previously unused in the series, had invaded the stage at the 2006 BAFTA Television Awards while the production team were collecting an award. A similar moratorium would be placed on the following series' finale, "Last of the Time Lords".
The episode's finalised average viewing figure was 8.22 million viewers and was, excepting World Cup games, the second most-watched television programme of the week, behind an episode of Coronation Street, and eighth most-watched overall. The companion episode of Doctor Who Confidential gained just over one million viewers, making it the second most watched programme on a non-terrestrial channel that week. The ratings for the episode were higher than the following World Cup match between Germany and Portugal, which had a million fewer viewers.
"Doomsday" is one of the most popular episodes of the revived Doctor Who. It gained an audience Appreciation Index (AI) of 89, which was the highest figure for nearly two years—it was later surpassed by "The Stolen Earth", which had an AI of 91—and is the first episode of Doctor Who to receive a perfect 10 rating on IGN, who congratulated Davies on making an action-packed episode so emotional.Television Without Pity gave the episode an A+ rating.The Stage commented that the Dalek-Cybermen conflict was the "only thing worth watching" at the weekend, overshadowing even the World Cup Final, and that the parting scene was "beautifully written and movingly played," with "not a dry eye in the universe". Dek Hogan of Digital Spy stated that the episode was "beautifully balanced and with moments of high excitement and touching poignancy" and that the single oil tear shed by the Cyberman version of Hartman was a "nice touch". He criticised Catherine Tate's appearance as being unnecessary to end the episode and for "breaking the mood". Stephen Brook of The Guardian thought that the episode was "a highpoint of the modern series, highly emotional, scary and genuinely exciting", while Rose's departure was "brilliantly handled". He positively compared the episode's plot of a war between "the greatest monsters in the programme history" against the film Alien vs. Predator.