Following the broadcast of the 2005 series, executive producer Russell T Davies explained that the real life United Nations were no longer happy to be associated with the fictional organisation, and the UN's full name could no longer be used. However, the "UNIT" and "UN" abbreviations can be used, as long as it is not explained what the letters stand for. In 2008, he announced that the organisation's name has been changed to the "Unified Intelligence Taskforce". This new name was first mentioned on-screen in "The Sontaran Stratagem".
The second incursion, as seen in the Second Doctor serial The Web of Fear (1968), 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. Another small group of British infantrymen, led this time 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.
Following the Yeti Incident, the United Nations became aware that the world faced threats from extraterrestrial sources, and that with the space program 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 is given jurisdiction over first contact situations, as revealed in The Sound of Drums. Lethbridge-Stewart was promoted to the rank of Brigadier and put in charge of the British contingent, which was apparently under the purview of the British government's Department C19. Department C19 was first 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.
The newly formed UNIT's baptism of fire was an invasion by the Cybermen, in the 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 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).
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.
When the Third Doctor's exile was lifted, his association with UNIT became more sporadic, especially after his regeneration into his fourth incarnation. The last appearance of UNIT in the series for many years was in The Seeds of Doom (1976); however, the organisation continued to execute its mandate to investigate and combat alien activity.
Lethbridge-Stewart retired in 1976 (Mawdryn Undead, 1983), and was succeeded by Colonel Crichton (The Five Doctors). UNIT did not appear again in force until the Seventh Doctor serial, Battlefield (1989), where the British contingent (although it also has foreign members) was commanded by Brigadier Winifred Bambera, and Lethbridge-Stewart was called out of retirement to help defeat an other-dimensional invasion of armoured knights led by Morgaine.
UNIT was referenced by acronym and full name in the 2005 series episodes "Aliens of London" and "World War Three", where it sent a delegation to a gathering of experts at 10 Downing Street in response to a spaceship crashing in the River Thames. All of the experts were electrocuted by the alien Slitheen. None of the members of UNIT seen were from the original series, although one of them was originally said to be Doctor Who Magazine comic strip character Muriel Frost. They would appear again in "The Christmas Invasion"', with a facility in the Tower of London, access to alien language translation software, and awareness of Martians. Prime Minister Harriet Jones oversaw the Sycorax crisis from this facility alongside commanding officer Major Blake.
The UK contingent of UNIT has ties to Torchwood, while the United Nations are unaware of its existence; this may indicate that the UK contingent of UNIT keeps secrets from its parent organisation. Major Blake contacted Torchwood to assist against the Sycorax in "The Christmas Invasion"; in Torchwood episode "Greeks Bearing Gifts", Jack Harkness mentions putting together some documents for UNIT; in "End of Days", UNIT is one of the groups that have contacted Harkness about the events of that episode. There is rivalry between the two groups however; in "Reset", Jack derisively refers to UNIT in this episode as "decaffeinated Torchwood" and "the acceptable face of intelligence gathering about aliens", and Torchwood did not inform UNIT about the powerful Resurrection Gauntlet.
"Turn Left" revealed that UNIT was involved in investigating the attempted Racnoss invasion, independently of the Doctor. In the alternative universe created in that episode by the Tenth Doctor's death, one UNIT team discovered the Doctor's body, whilst another under Captain Marissa Magambo and Rose Tyler salvaged "surface technology" from the Doctor's dying TARDIS in order to send Donna Noble back in time and prevent the Doctor's death.
In "The Sound of Drums", UNIT is shown to have an aircraft carrier called the Valiant designed by Minister of Defence and later Prime Minister Harold "Harry" Saxon (alias the Master). UNIT assumes control of handling the Toclafane visitation, not knowing it has been secretly engineered by the Master. While brief radio reports can be heard near the end suggesting UNIT is being overwhelmed by the Toclafane invasion, the Paradox machine's destruction reverses time to just before the invasion began.
In the The Sarah Jane Adventures serial "Revenge of the Slitheen", Sarah Jane Smith telephones UNIT to tell them about the secret rooms around the world with alien machinery inside, located in schools constructed by the fictional Coldfire Construction. UNIT is also referred to in the serial "The Lost Boy", where UNIT used its political clout to pull strings with the London police to have its former quasi-member Sarah Jane Smith (Sarah was never an official employee of UNIT in the way that her two predecessors were) released without charge after she was arrested for alleged child abduction.
In the Torchwood episode "Reset" it is established that the Doctor's former companion Martha Jones has joined UNIT as a qualified doctor (the Doctor having recommended her to UNIT) and when Jack Harkness was in need of some help he drafted her into the Torchwood Institute on a temporary basis. UNIT were working on the same mystery as the Torchwood Institute in that episode and the two organisations pooled their resources in order to solve it.
Beginning in the later Torchwood episode "Fragments" UNIT is depicted as having little regard for law or human rights. After a young Toshiko Sato becomes unwillingly involved with terrorists, she is captured by UNIT soldiers and subject to an indefinite term of imprisonment without trial, until she is rescued by Jack Harkness and recruited to Torchwood.
UNIT's first proper team-up with the Doctor in the new series occurred in the 2008 2-parter "The Sontaran Stratagem/The Poison Sky". They are now a larger, better-outfitted organisation, getting a large amount of legal powers (and of funding from the United Nations) — including the capability to command & co-ordinate the planet's nuclear weaponry in a single strike — in the name of "Homeworld Security". The UK branch is under the command of Colonel Mace. Under the codename Operation Blue Sky, UNIT (via Martha Jones) called in the Doctor and seized control of the central factory for ATMOS Systems, intending to investigate whether it was an alien front organisation. In the process, two soldiers were brainwashed by the Sontarans and Martha Jones replaced by a clone, while the Sontaran Tenth Battle Fleet (reacting to the Doctor's presence) advanced their invasion plans, attempting to change the atmosphere and disabling UNIT's nuclear strikes. Despite an initial massacre at the ATMOS factory, a change in weaponry and strategic use of the Valiant meant that UNIT retook the factory and defeated the Sontaran attack force there, giving the Doctor the opening to stop the Sontaran stratagem.
"The Stolen Earth"/"Journey's End" two-parter showed that a major UNIT base in Manhattan had managed to create a rudimentary teleport device based on salvaged Sontaran technology, known as Project Indigo; Martha Jones had been promoted to Indigo's chief medical officer. UNIT had also created the Osterhagen Key, a doomsday weapon that would trigger over twenty nuclear warheads under the Earth's crust and destroy the planet in case an invasion left humanity in incredible suffering with no hope. When the Earth was shifted to the Medusa Cascade by Davros and the Daleks, the Daleks attacked UNIT bases, destroying the Valiant and wiping out the Manhattan base. While the UN surrendered, Martha was ordered to use Indigo to escape, attempt to find the Doctor and (if no other option remained) use the Osterhagen Key. With her help, the Daleks were defeated and Earth returned, and it was hinted she may have destroyed the Key.
In The Invasion, UNIT had a command centre established in the cargo hold of a C-130 Hercules military transport aircraft. The Dæmons featured the UNIT Mobile HQ, a large bus-like vehicle that could be driven to the site of an incident. A mobile command centre is also shown in "The Sontaran Stratagem" and "The Poison Sky", where it is depictied as a black Articulated Lorry with UNIT insignia.
Prominent members of the British contingent of UNIT included Liz Shaw, Sergeant Benton, Captain Mike Yates, Jo Grant and later, Harry Sullivan. Civilians who have worked with UNIT include the journalist Sarah Jane Smith. In The Claws of Axos (1971), an American agent named Bill Filer was sent from Washington to assist in the hunt for the Master.
In "The Christmas Invasion", UNIT is shown to have translation software which can decipher alien languages with great accuracy. The software, or at least the results from the translation, can be loaded on a hand-held device. In "The Sound of Drums", the flying aircraft carrier Valiant is introduced.
In "The Poison Sky", UNIT is shown to be able to command and co-ordinate the world's combined nuclear arsenal for strategic strikes on orbiting alien craft. The Valiant is also shown to be equipped with a scaled down version of the Torchwood weapon that destroyed the Sycorax ship in "The Christmas Invasion".
Both Virgin Publishing's Missing Adventures and BBC Books' Past Doctor Adventures have set stories in the UNIT era and have revealed new information about UNIT's past, present and future. The New Adventures novel Just War by Lance Parkin mentions LONGBOW, a world security organisation set up by the League of Nations that encountered the occasional extraterrestrial incident but was disbanded after it and the League failed to prevent World War II. No Future by Paul Cornell featured an intelligence section of UNIT called Broadsword. Broadsword agents wore plain clothes and were "hand-picked to offer us lateral and non-military solutions, backed up by SAS training and sheer common sense". Since the novel was set in an alternate 1970s with subtle differences from the main continuity Whoniverse, it is not clear if Cornell intended Broadsword to exist in normal continuity.
The Dying Days, also by Parkin named the French division of UNIT as NUIT (Nations Unies Intelligence Taskforce), and the Eighth Doctor Adventure Emotional Chemistry by Simon A. Forward named the Russian division ОГРОН (OGRON) (Оперативная Группа Разведкой Объединённых Наций, or, Operativnaya Gruppa Rasvedkoy Obyedinyonnih Natsiy, which roughly translates as "United Nations Reconnaissance Operations Group"). The Southeast Asian contingent was identified in David A. McIntee's Bullet Time as UNIT-SEA.
The Devil Goblins from Neptune by Keith Topping and Martin Day introduced a division within the Central Intelligence Agency headed by a man known only as Control, which has featured as a rival to UNIT in several subsequent novels. Alien Bodies by Lawrence Miles introduced a more ruthless UN division called UNISYC (United Nations Intelligence Security Yard Corps), which by the 2040s has replaced UNIT. By the 26th century, UNIT has transformed into a secret society called the Unitatus, pledged to defend the Earth against alien threats, first seen in Parkin's Cold Fusion. The Unitatus lasts at least until the 30th century (So Vile a Sin by Ben Aaronovitch and Kate Orman).
UNIT has also appeared in cameo roles in unrelated comics. In at least one issue of Uncanny X-Men, where a character identified as Brigadier Lethbridge-Stewart was seen briefly from behind, addressing a Sergeant-Major Benton; 2000 AD's Caballistics, Inc. strip has Lethbridge-Stewart (referred to solely by rank) appearing in several adventures as a military liaison and referring to The Web Of Fear; and Hip Flask has a 22nd Century UNIT tied into the origins of the Elephantmen. Marvel Comics also has two major characters called Dr Alistaire Stuart (who has claimed to know "a chap from Gallifrey") and Brigadier Alysande Stuart, Scientific Advisor and commander respectively of Britain's Weird Happenings Organisation (W.H.O.) taskforce. W.H.O. has since been disbanded and Alysande killed, but Alistaire Stuart is still a recurring character in Marvel's United Kingdom.
In December 2004 Big Finish released UNIT: Time Heals, the first of a new series of UNIT audio plays, featuring a retired General Sir Alistair Lethbridge-Stewart as an advisor to a new generation of officers. A preview episode (given away free with Doctor Who Magazine #351), UNIT: The Coup, had Lethbridge-Stewart finally breaking decades of secrecy by informing a press conference of UNIT's true purpose as humanity's first line of defence against the unknown (although, as it turned out, the general public believed this to be a hoax). The series also introduced another rival division, this time within the British government, the Internal Counter-Intelligence Service, or ICIS. In September 2008, The Coup was re-released as a free download.
The protagonists for most of this series were Colonel Emily Chaudhry (a public-relations specialist played by Siri O'Neal), Lieutenant Will Hoffman (Robert Curbishley) and Colonel Robert Dalton (a veteran of the British Army temporarily assigned to UNIT, played by Nicholas Deal). Hoffman and Dalton were killed in the third instalment, UNIT: The Longest Night. The fourth and so far final play, UNIT: The Wasting, features this Universe’s version of Brimmicombe-Wood (again played by Tennant). The short story "The Terror of the Darkness" in the collection Short Trips: A Day in the Life revealed that Chaudhry and Hoffman had previously travelled with the Sixth Doctor. Their adventures then continued in "Incongruous Details" (Short Trips: The Centenarian) before ending in Short Trips: Defining Patterns.
BBV have made a trilogy of UNIT videos involving the Autons, although they feature none of the original members with the main character being a psychic operative named Lockwood. The trilogy introduced one of UNIT's facilities (the Warehouse) for containing the remains of alien technology; the Containment Team responsible for these facilities and preventing alien outbreaks at them; and the Internal Security Division.
For the new television series, BBC created a faux website for UNIT at www.unit.org.uk, complete with "easter eggs" that can be accessed by the reader with the passwords "bison" and "buffalo" (the latter mentioned on screen in "World War Three"). The 'public' part of the website advertises UNIT Conferences and publications relating to "extra-territorial threats", as well as press releases on the establishment of a central New York Liaison office; the press releases and publications also make reference to off-screen adventures, such as the Skaniska Incident and Jersey Tollgate Situation, with the most recent covering the events of The Christmas Invasion ("Alien Life Confirmed"). The Secure Login link, using the password "badwolf" (originally "bison") uncovers a 'private' section which provides UNIT point-of-view reports about various events in the 2005 series, as well as mention of missions such as The Fourth Reich and Guatemala "Big Locust" Problem. The website's canonical status, like that of the spin-off media, is debatable. Due to the objections by the United Nations as noted above, the letters "UN" are no longer expanded to "United Nations" on the website.
The original 1963-1989 series presented conflicting evidence about when the stories featuring UNIT were meant to take place, and there has been much confusion and continuing fan debate on this subject. Although there is strong evidence that at least some of the production team intended for the UNIT stories to take place in the "near future", this policy was not consistently applied and there is equally strong evidence to suggest the stories took place at the time of their broadcast. Whether the stories take place contemporaneously with the broadcast dates or a few years in the future is therefore highly debatable — with the exception of Battlefield, which is explicitly set in an unspecified near future. A reference to this confusion appeared in the 2008 episode The Sontaran Stratagem, where the Doctor was unsure if his time on the UNIT staff took place during the '70s or '80s.