The Invasion is the third serial of the sixth season of the British science fiction television series Doctor Who, which was first broadcast on BBC1 in eight weekly parts from 2 November to 21 December 1968. In the serial, Tobias Vaughn (Kevin Stoney), the head of a successful electronics company, forms an alliance with the Cybermen to conquer Earth. The Doctor (Patrick Troughton) and his companions, Jamie McCrimmon (Frazer Hines) and Zoe Heriot (Wendy Padbury), team up with UNIT, headed by Brigadier Lethbridge-Stewart (Nicholas Courtney), to combat the invasion.

The serial was conceived by Kit Pedler, after another Cyberman story was ordered by Peter Bryant during the production of The Wheel in Space (1968), a part of the fifth season. A full script was then written by Derrick Sherwin, which was intended to be a follow-up to The Web of Fear (1968), another fifth season serial. Due to the lower budget required for an action based story set on Earth, the serial was envisioned as a pilot to the Third Doctor's Earth based serials. Directed by Douglas Camfield, filming took place between 30 August and 8 November 1968 with two weeks being dedicated to extensive location shoots. The serial marks the first appearance of UNIT and the return of Lethbridge-Stewart from The Web of Fear, both of which would become recurring elements in the show.

The serial averaged 6.9 million viewers, which was a slight increase from the previous serial. The Invasion has received a generally positive reception, with reviewers praising the serial's atmosphere and direction, Stoney's performance, the Cybermen costumes, and Don Harper's score, although the serial's length has received mixed reactions. In 1971, the master tapes of all episodes were ordered to be junked, causing episodes 1 and 4 to be missing from the BBC's archives. While the other 6 episodes survive, some stills and full audio recordings of the missing episodes exist. The story has been novelised by Ian Marter and has been released on VHS, DVD, and as an audiobook. In 2006, it became the first incomplete Doctor Who serial to receive official full-length animated reconstructions of its two missing episodes based on the pre-existing material.

Plot The TARDIS appears above the dark side of the Moon, whereupon a rocket is fired upon it. The Doctor, Jamie and Zoe land the damaged TARDIS in the countryside and go to find Professor Edward Travers in London for his assistance. They discover Travers has leased his house to Professor Watkins and his niece Isobel. Watkins has gone missing while working for a shadowy electronics company called International Electromatics. The Doctor and Jamie leave to investigate its head office, where they meet Tobias Vaughn, the company's Managing Director, and reunite with a promoted Brigadier Lethbridge-Stewart, head of a military taskforce called UNIT, which investigates unusual activities around the world.

Taken to the company's countryside base, the Doctor and Jamie meet Watkins, who is working on a "Cerebration Mentor" device, which can instill specific emotions. Watkins reveals that Vaughn is working with an unspecified ally and that they are planning to take over the world. Further investigation reveals this ally as the Cybermen, who intend to send a hypnotic signal through the devices produced by International Electromatics, which will incapacitate the world's population and nullify resistance. The Doctor is able to protect his companions and their UNIT allies with specially made depolarisers that neutralise the Cybermen's signal, which were created by a newly freed Watkins. The Cybermen take over, the group flees Vaughn's soldiers, who attempt to recapture Watkins.

After completing production on more depolarisers, the Doctor leaves to confront Vaughn in London whilst UNIT works to stop the Cybermen. Uncovering Russian plans to launch a rocket at the ship sending the signals, Turner leads a squadron to assist them whilst Zoe helps the Brigadier predict the Cyberfleet's movements. Using British artillery, they are able to destroy the full fleet, causing the Cybermen to turn on Vaughn and decide to destroy Earth with a megatron bomb. His plans to preemptively betray the Cybermen by using the Cerebration Mentor against them ruined, Vaughn agrees to thwart the invasion and helps the Doctor locate the homing signal. With UNIT sending troops to help, they defeat the Cybermen guarding the beacon and turn it off, though Vaughn is killed in an ambush. The megatron bomb is destroyed by an anti-missile defence rocket, while the Russian rocket destroys the Cybership broadcasting the hypnotic control signal, ending the invasion. Following the success, the Doctor and his companions leave in the repaired TARDIS.

Production Conception and writing During the production of The Wheel in Space (1968), Peter Bryant commissioned a fifth Cyberman serial from Kit Pedler following their positive reception in previous serials. The developed serial was intended to be a six-part story under the working title The Return of the Cybermen, a name shared with the working title of The Moonbase (1967). Over the summer, the serial would be retitled to keep the appearance of the Cybermen a surprise. The Radio Times, however, would reveal the twist prematurely. Bryant felt that the outline could only fill 4 episodes. Still, he paid Pedler on 14 May for the rights to the outline, the Cybermen and Cybermats. However, the latter would not appear in the final serial. Pedler also created Tobias Vaughn, whose name was inspired by a book of Christian names that Pedler owned. A full script was intended to be finished by David Whitaker, who also wrote The Wheel in Space, but he had commitments to other shows. Pedler's collaborator Gerry Davis was also approached, but was busy as the script editor for The First Lady. Derrick Sherwin, the show's leaving script editor, was hired by Bryant to finish the script and expand it to a total of 8 episodes by 1 June, while assistant story editor Terrance Dicks became the full-time script editor. Additional rewrites of the script occurred from 10 May to 26 August. Inspired by the Quatermass series (particularly Quatermass II), the goal of the scripts was to write a serial focused on action, which would be based on Earth; Sherwin and Bryant intended this to serve as a basis for the show's next season, which could then be produced on a lower budget. The BBC approved the first five episodes on 11 June, with the last three being approved on 27 August.

The serial was intended to mark the return of Professor Travers, Lethbridge-Stewart and Driver Evans, all of whom appeared in The Web of Fear (1968). Evans was excluded as the BBC believed him to be a harmful stereotype of the Welsh by implying that they were cowards. The Web of Fear writers Mervyn Haisman and Henry Lincoln agreed in May 1968 to the use of Travers and Lethbridge-Stewart, despite ongoing disputes between the BBC and both writers over rewrites to The Dominators (1968). Travers was replaced by Professor Watkins, due to Travers' actor, Jack Watling, costing too much for the role he would have in the story; Rewrites had reduced the character's prominence, which caused the BBC not to offer him the role. Anne Travers, Travers' niece, would be replaced by Isobel, who was envisioned as a possible companion for a season set on Earth. Older sources, however, indicated that Watling and Tina Packer, Anne's actress, were not available to reprise their roles. Only Lethbridge-Stewart was kept in the story, as replacing him with a similar character would allow Haisman and Lincoln to claim ownership over the derivative character, and the writers were paid () per episode for Lethbridge-Stewart's use. The character would be promoted to the rank of brigadier and placed as the head of UNIT, which was conceived by Sherwin to be part of the formula of the following season. UNIT would go on to be a recurring presence in the classic series (being a key element of the Third Doctor's era) and the new series. The director of The Web of Fear, Douglas Camfield, would be rehired to help bring a similar atmosphere to the project.

Music and design

Australian composer Don Harper was hired to compose the incidental music for the serial in his only work on Doctor Who. Camfield's refusal to use regular composer Dudley Simpson led to Harper being commissioned. Simpson believed Camfield did not employ him because of a dispute the two had at a dinner party about Simpson's earnings. However, Camfield claimed he did not want to have Simpson working too long on the same show. Harper had previously worked with Camfield on Out of the Unknown in May 1968. Harper then asked Simpson for advice for his gig, which Simpson gave without being aware of what show Harper was working on. Initially the music was planned to be recorded on 30 August, however the recording was rescheduled. Harper ultimately composed 20 minutes of cues, of which 13 minutes were used; the cues were recorded on 16 September with his ensemble. Though uncommon at the time, the music was pre-recorded for the music to last through several scenes, even when the scenes were shot out of order. The music was supplemented with tracks by John Baker for Out of the Unknown and Henry Hall, as well as stock music. Brian Hodgson composed several sound effects to be used in the serial, as well as some supplemental music in Harper's style.

Originally, the Cyberman were meant to reuse the costumes produced in 1967, but due to most of the old costumes being unusable and the serial requiring large groups of Cybermen, new costumes would be created. Costume designer Bobi Bartlett, in her first work for the programme, redesigned them to appear more militaristic and "tough", with 6 new costumes being assembled. The Cybermen's helmets were given an "earmuff" shape, had a built-in torch, and were made of fibreglass. The helmets were more comfortable, since they were a looser fit to increase air circulation and prevent fainting, which had been a problem during The Moonbase. The rest of the costume being based on a two-piece wetsuit, army boots and gloves, all painted grey, though the paint would crack during production. The chest devices were also redesigned to be smaller and contain battery powered lights. Heat caused the costumes to become sweaty to the point that pints of sweat would be poured from the boots over the course of filming, with one actor's boots audibly squelching during production. Additionally, the actors' movements loosened the metallic rods on the bodysuits, a problem also encountered with the costumes in The Wheel in Space, which caused an injury during filming. Camfield was advised for the design of the uniforms of UNIT soldiers, which like other costumes were designed to be slightly futuristic, because of his former experience in the military. Camfield disliked the costumes and avoided them for his following serial.

The serial's sets were designed by Richard Hunt, who had previously worked on three Doctor Who serials: Galaxy 4, "Mission to the Unknown" (both 1965) and The Smugglers (1966). The Invasion was taken as an experiment by Hunt and Camfield to use the limited space in Lime Grove's Studio D more efficiently. Sets would be built in front of each other and taken apart during breaks in filming to use the sets behind. The Design Department was required to build 17 sets, with the first 3 episodes requiring 7 each, causing internal panic. Make-up was done by Sylvia Jones, who had worked on the job since The Abominable Snowmen (1967). The serial featured a large number of firearms, which required an armourer; since all of the armourers from the Props Department were busy or on vacation, a contractor was hired. The Cybermen's guns were designed to shoot smoke charges, which were only used in location shoots due to safety regulations.

Casting