Throughout the history of Doctor Who, the Doctor has come up against many enemies and many allies. Daleks, Torchwood, Cybermen, and UNIT all have roots woven throughout the run of the show, frequently making appearances and clashing with The Doctor in one way or another.
When it comes to UNIT (short for United Nations Intelligence Taskforce), they are perhaps the Time Lord's most consistent ally over the course of the series. But what is the mysterious military operation, and what part does it play in the past and the future of the show?
In The Classic Series
Considering the show's relationship with time travel, it should come as no surprise that the story of UNIT is told in a non-linear fashion. UNIT was brought into existence to deal with the increasing extraterrestrial encounters that Earth was experiencing. However, it was preceded by the Intrusion Counter-Measures Group, helmed by Captain Ian Gilmore. The group became involved in the Dalek civil war in the episode "Remembrance of The Daleks," a Seventh Doctor story that aired in 1988 but was set in 1963. The group, led by Gilmore with scientific advisor Dr. Rachel Jensen and her assistant Allison Williams, investigated magnetic fluctuations and became involved in the Dalek fight. Ultimately the force facilitated the evacuation of the people in the area and helped to ensure The Doctor could carry out his plan.
From this first appearance, the Intrusion Counter-Measure Group was used as a sort of proof of concept within the narrative show itself to introduce the creation of UNIT. However, UNIT had been a part of the Doctor Who mythos for many years before its origin was shown onscreen.
UNIT was officially formed in either 1969 or 1971, according to different accounts in expanded media. The military operation was created by the UN to monitor, investigate, and combat any extraterrestrial threats that may arise. Corporal Alistair Gordon Lethbridge-Stewart was brought on board to head the British division of the operation, ultimately earning him a promotion to the rank of Brigadier.
The first appearance of UNIT was in season 6 of the show, in the episode "The Invasion." This Second Doctor story focused on the Cybermen and their attempt to take over London. UNIT was investigating International Electromatics, and ultimately discovered that the company was a front for the Cybermen invasion. With the help of the Doctor, UNIT was able to stop the invasion but not without great violence and losses.
Later in the series, when the Third Doctor was exiled to Earth by the Time Lord council, he agreed to join UNIT as a scientific advisor at the request of the Brigadier, who believed a scientifically-based approach to counteracting extraterrestrial threats was necessary. Despite this collaboration, the Brigadier and the Doctor didn't always see eye to eye. In "Doctor Who and The Silurians," the Brigadier authorized his people to blow up the underground home of the titular creatures, permanently sealing them in. This was not the approach that the Doctor had wanted to take, and he objected throughout the situation.
Despite this, UNIT and The Doctor continued to collaborate, and the Doctor even had his own room in the headquarters of the operation. Throughout his time in exile, the Doctor assisted UNIT many times, but when companion Jo decided it was time to depart UNIT, his involvement with the operation decreased considerably. However, in "Planet of the Spiders," UNIT aided the Doctor in researching ESP and the Eight Legs, a race of spiders intent on feeding on human meat. Ultimately, the Doctor sacrificed his life to stop them, and upon being fatally poisoned, returned to UNIT to regenerate in front of the Brigadier and Sarah Jane Smith.
The Fourth Doctor had even less interaction with UNIT, although he and Sarah Jane Smith did assist them and vice versa on a few occasions. After the events of "The Seeds of Doom," where they investigated Krynoid seed pods in Antarctica, The Doctor ceased working with the operation, but he never officially left the organization. The Brigadier retired from UNIT in 1976 to teach mathematics and was replaced by Colonel Charles Crichton.
From this point, UNIT doesn't appear in the show again until season 26 with The Seventh Doctor in the "Battlefield" story. It was also the Brigadier's first appearance in the show since "The Five Doctors." After this appearance UNIT wasn't seen again in the classic series. There have been many audio adventures involving UNIT detailing various adventures and leadership changes, but it's unclear how much of this is actually canon.
In The Revived Series
UNIT made its reappearance after the show was revived in 2005. During the Ninth Doctor's run, UNIT was mentioned during the Slitheen's attempted takeover during "Aliens in London." They didn't play a larger part, however, until "The Christmas Invasion," after The Doctor regenerated into Ten. During this episode, newly elected Prime Minister Harriet Jones makes contact with UNIT and utilizes them and Torchwood to counter the Sycorax.
The Tenth Doctor didn't like the way in which UNIT was changing its stance on civil rights, nor its employment of a secret prison where prisoners were kept without charges. When Prime Minister Harold Saxon took over, UNIT played a much larger role, operating the Valiant air carrier and making first contact with the Toclafane. Of course, Harold Saxon was actually The Master, and the Toclafane were not a friendly alien contact. After these events, the Doctor recommended that UNIT recruit Martha Jones, his companion who had recently stopped traveling with him. UNIT took his advice, and Martha worked with them for a time before becoming disillusioned and becoming a freelancer of sorts.
Since then, UNIT has remained a presence in the show, but mostly in the background, or being limited to mentions. It is shown that UNIT is now under the leadership of Kate Stewart, the daughter of the Brigadier. Kate steered the organization away from its more militaristic views and back towards science, helping the Twelfth Doctor deal with threats such as Missy's army of Cybermen and the invasions of the Zygons.
By 2020, the operation seemed to have been shut down by Prentis, who was actually revealed to be The Grand Serpent. During the Flux event, Kate led the human resistance against the Sontarans and made herself an enemy of Prentis in the process. Ultimately the humans and The Doctor prevailed against the Sontarans and the Flux, leaving the future of UNIT and Kate Stewart up in the air. With the Fourteenth Doctor recently revealed, it remains to be seen what will become of UNIT, and what kind of relationship Fourteen will have with them.