UNIT (UNified Intelligence Taskforce, formerly United Nations Intelligence Taskforce) is a fictional military organization from the British science fiction television series Doctor Who. Operating under the auspices of the United Nations, its purpose is to investigate and combat paranormal and extraterrestrial threats to the Earth.
As is common in long-running series whose backstories are not mapped out in advance, and are also the product of many different writers over the course of years, the fictional history of UNIT have seen retroactive changes which have caused some continuity problems.
Pre-UNIT
Following the canon of the television show only, the roots of the organization in the history of the Doctor Who universe lie in one extraterrestrial incursion. As seen in the Second Doctor serial The Web of Fear (1968), there was an attempt to take over London by a disembodied entity known as the Great Intelligence, using robotic Yetis and a deadly cobweb-like fungus. A small group of British infantrymen, ultimately led by Colonel Alistair Gordon Lethbridge-Stewart of the Scots Guards (assisted by the Doctor), beat back the attempted conquest in the tunnels of the London Underground.
According to several sources outside of the television programme itself, UNIT also owes part of its existence to a much later television episode, the Seventh Doctor serial Remembrance of the Daleks (1988). In that incident, two Dalek factions fought a battle in London over the Time Lord artefact known as the Hand of Omega in late 1963. They were defeated by detachment of soldiers from the 'Intrusion Counter-Measures Group', commanded by Group Captain "Chunky" Gilmore, along with help from the mysterious time traveller known as the Doctor. Gilmore also had the assistance of a Scientific Advisor, Dr. Rachel Jensen. According to non-canon sources, the ICMG was a special anti-terrorist group which drew its forces from the regular Army, and also the RAF Regiment. The Dalek incident was of course covered up. The ICMG was disbanded shortly afterwards; however, several of its training materials and procedures were adopted by UNIT. Gilmore later served as an advisor, often lecturing for UNIT personnel.
Following the Yeti Incident, the United Nations became aware that the world faced threats from extraterrestrial sources, and that with the space programme sending probes deeper and deeper into space, mankind had drawn attention to itself. Consequently, the United Nations established UNIT with the mandate to investigate, monitor and combat such threats. The United Nations was also given jurisdiction over first contact situations in 1968, as revealed in "The Sound of Drums". Lethbridge-Stewart was promoted to the rank of Brigadier and put in charge of the British contingent of UNIT, which was apparently under the purview of the British government's Department C19. Department C19 was mentioned in the serial Time-Flight, being the department at whose behest the Fifth Doctor investigated the mystery of a Concorde aeroplane that had disappeared. Several of the spin-off novels explore the idea that C19 gathers up alien technology for their own ends, as revealed in The Scales of Injustice and Who Killed Kennedy. The canonicity of the novels in relation to the series is unclear.
Twentieth century
Four years after the incident in the London Underground, the newly formed UNIT's baptism of fire was an invasion by the Cybermen, in The Invasion (1968). UNIT repulsed this, once again with the Second Doctor's help. Following this, Lethbridge-Stewart became convinced of the necessity of scientific advice in battling extraterrestrial threats, and recruited Dr Elizabeth Shaw from Cambridge. Coincidentally, the recently regenerated Third Doctor had been exiled to Earth by the Time Lords, and he agreed to join UNIT as its Scientific Advisor just in time to help defeat the Autons (Spearhead from Space). The Doctor was later assisted by Jo Grant.
In addition to combating alien threats, the British contingent has also been responsible for providing general security under the protection of the UN. A significant example was the provision of security at the Styles peace conference.
UNIT first operated out of an office building in London and subsequently moved to a headquarters in the country that had been built over the ruins of a priory (Pyramids of Mars). Its main headquarters, mentioned but never seen in the television series, is with the United Nations in Geneva.