This article contains spoilers for the Doctor Who Anniversary Special, "The Star Beast"

The organization called UNIT has been a mainstay inDoctor Who since 1968, and is now bigger than ever in Russell T. Davies' latest relaunch. Alien invasions are a frequent occurrence inDoctor Who. Unsurprisingly, that means the world has responded, withthe United Nations coordinating efforts to deal with extraterrestrial threats. The organization known as UNIT has, of course, crossed paths with the Doctor many times.

UNIT was officially disbanded during the Thirteenth Doctor's era, although it reformed by "The Power of the Doctor." Now, according to "The Star Beast,"UNIT is back and bigger than ever. Here's all you need to know about UNIT, the organization's leaders, and the Doctor's relationship with this top-secret military group who have somehow been operating in plain sight for decades.

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UNIT Protects The Earth From Extraterrestrial Threats

The story of UNIT begins with the 1968 story "The Invasion." There, Patrick Troughton's Second Doctor visited Earth only to learn humanity had responded to frequent alien investigations by establishing a new military taskforce to protect the planet. Originally named theUnited Nations Intelligence Taskforce, the British wing of UNIT was run by the Doctor's allyAllister Gordon Lethbridge-Stewart (played by the late Nicholas Courtney). UNIT suffered something of a baptism of fire when the Cybermen invaded London, and over the next few years, they faced a steady stream of invaders. The modern UNIT is officially called theUnified Intelligence Taskforce (the name changed because, in the real world, the U.N. objected to the name and logo).

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UNIT operate in the open, although for most of their lifespan,their true mission has been cloaked in secrecy. Recruits are drawn from the conventional military, and they soon learn they are dealing with something far more dangerous than any conventional threats. Fortunately, UNIT's experience means they have developed the technology to deal with many different alien races, and by 2009's "The Sontaran Stratagem"they even had a helicarrier as a base. During the Thirteenth Doctor's era, allies of the Sontarans who sought to invade the Earth worked to disband UNIT for a time.

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Who Is "Kate," UNIT's Current Leader?

"The Star Beast" name-drops UNIT's current leader,Kate Lethbridge-Stewart, played by Jemma Redgrave. The daughter of the Doctor's old friend Brigadier Lethbridge-Stewart, Kate spent most of her formative years unaware of her father's heroism - and frustrated because he was so rarely at home. Kate only learned the truth when an alien entity known as the Great Intelligence attempted to get revenge on Brigadier Lethbridge-Stewart by using her against him. She subsequently joined UNIT, although she initially changed her name - removing "Lethbridge" - so she could earn respect based on her own achievements rather than stand on her father's shoulders.

Kate is thoroughly committed to the defense of the Earth, working alongside several recent incarnations of the Doctor herself. Suspicious when allies of the Sontarans engineered UNIT's shutdown during the Thirteenth Doctor's era, she slipped into the shadows and continued to operate in secret. UNIT was reestablished by the time of "The Power of the Doctor," when she called on several of the Doctor's past companions for help.

The Doctor Was UNIT's First Scientific Adviser

The Doctor was UNIT's first official scientific adviser, joining their payroll when he was exiled to Earth by the Time Lords. Jon Pertwee's Third Doctor worked closely with UNIT over the next few years, and subsequent incarnations of the Doctor took advantage of the position as well. Tom Baker's Fourth Doctor stepped down from the role shortly before Brigadier Lethbridge-Stewart's retirement, but he remained something of a legend.

UNIT has had many other scientific advisers over the years, including Caroline John's Liz Shaw, a Cambridge professor, Ingrid Oliver's Osgood, and even Kate Stewart herself. "The Star Beast" introducedRuth Madeley as Shirley Ann Bingham, UNIT's 56th scientific adviser. This alone indicates just how hazardous a role it is, because few of these scientific advisers have likely survived to retire - or to return to their original careers.

UNIT Work With The Doctor's Former Companions & Closest Friends

The Doctor has worked with many different companions over the decades, and UNIT has monitored them all. This finally became something of a plot point in "The Power of the Doctor," whenKate Lethbridge-Stewart called on two past companions - Janet Fielding's Tegan Jovanka and Sophie Aldred's Ace - for help. She subsequently formed acompanion support group - with the various former companions helping one another recover from theiir experiences, while also serving as a useful resource for UNIT. Confirmed members of the companion support group include:

The Modern UNIT Is Bigger (& More Public) Than Ever

UNIT tend to operate in the open, although their true mission is traditionally a secret. That said, there's evidenceUNIT is less secretive than ever before, with a massive base in downtown London seen in trailers forDoctor Who's 60th anniversary specials. It's possible the frequent alien invasions have finally led humanity to accept there really are extraterrestrials; if so, UNIT may now be public knowledge. This may well mean UNIT wind up in the line of fire when aliens invade - because they now UNIT need to be taken out first.

Key UNIT Stories In Doctor Who History

UNIT has been a key part ofDoctor Who lore since 1968. That naturally means anyone who wants to check out the most important UNIT stories will find themselves reliving quite a surprising number of past episodes, meeting Doctors ranging from Patrick Troughton to Jodie Whittaker. Here are the key UNITDoctor Who stories to date:

• "The Invasion," featuring Patrick Troughton's Second Doctor. This is essentially UNIT's debut story, with the Doctor teaming up with the newly-formed UNIT against the Cybermen.
• "Spearhead From Space," which introduces Jon Pertwee's Third Doctor. This sees the Doctor exiled to Earth, and he becomes UNIT's scientific adviser.
• "Terror of the Autons," another Third Doctor story. This is notable for introducing classic companion Jo Grant, a key UNIT member. It is also the first story to feature the Master, originally played by Roger Delgado.
• "The Seeds of Doom," featuring Tom Baker's Fourth Doctor. This was the Doctor's last mission as UNIT's scientific adviser.
• "Battlefield," starring Sylvester McCoy's Seventh Doctor. Part ofDoctor Who's 30th anniversary celebrations, this brought back Brigadier Lethbridge-Stewart and introduced a new generation of UNIT heroes.
• "The Sontaran Stratagem" and "The Poison Sky," featuring David Tennant's Tenth Doctor. This brought UNIT back for the post-2005Doctor Who relaunch, making them far more impressive than ever before.
• "The Power of Three," a story starring Matt Smith's Eleventh Doctor. This introduced Kate Stewart as the new leader of UNIT.
• "The Day of the Doctor," part ofDoctor Who's 50th anniversary celebrations, which saw UNIT work with three incarnations of the Doctor - John Hurt's War Doctor, Tennant's Tenth Doctor, and Smith's Eleventh Doctor.
• Doctor Who season 13, a six-part story collectively known as "Flux," in which Kate Stewart played a key role in defending Earth alongside Jodie Whittaker's Thirteenth Doctor.
• "The Power of the Doctor," Jodie Whittaker's final story asDoctor Who's Thirteenth Doctor. This saw Kate Lethbridge-Stewart begin to work alongside the Doctor's companions for the first time, setting up the organization's exciting future.