Kate Stewart, also known as Kate Lethbridge-Stewart and nicknamed “Tiger” by her father, was the Chief Scientific Officer of UNIT. She was the daughter of Alistair Gordon Lethbridge-Stewart and his first wife Fiona, and the mother of Gordon Lethbridge-Stewart and another child.
Kate spent her youth not knowing of her father’s activities with extra-terrestrial beings, believing whole-heatedly that his life consisted of a boring and simple military job. She and her mother spent much time ironically joking that he was constantly off on amazing adventures when they both knew the “truth.” Kate would not begin to scratch the true surface of her father’s life until years later, when she shared an encounter against the Great Intelligence with him. She would spend some time some time investigating UNIT an her father’s ties to it, at one point encountering ex-UNIT operative Douglas Cavendish to investigate a “haunting,” which revealed itself to be a Dæmon. She soon joined the organization, rising to its top position and changing it to be a more scientific, peaceful group.
As the leader of UNIT, Kate encountered the Doctor many times during encounters between Earth and alien threats, becoming similar to the Doctor as her father had been. She was at the forefront of the Human-Zygon coalition, setting a new precedent for peace not previous seen in the organization against the Silurian threat.
Fiona and Alistair had Kate three years into their marriage, which lasted eight years. As a child, Kate was never told of her father’s work at UNIT with aliens and was only aware that he was a military man. She would often pretend that he was off to have an amazing adventure when she really “knew” he was off to do boring military operations.
At the time of the Wenley Moor Silurian incident, Kate was five. It was during this mission that Alistair realized that he was an inadequate father for her. The secretive nature of his work with UNIT prevented him from being consistently present in her life. His long absences from home caused Fiona to leave him. Alistair guessed that she and Kate went off to at least initially live somewhere close to Chichester, the home of Kate’s maternal grandparents.
Eventually, Kate grew estranged from her father. In her teens, Kate had a son whom she named Gordon Lethbridge-Stewart, thereby demonstrating a continued love of her father, despite her outward ambivalence and her decision not to reveal her pregnancy or Gordon’s existence to him. Gordon’s father, Jonathan, wanted to marry her, but she wanted to remain single. Jonathan and Kate split up when Gordon was two, leaving Kate to raise Gordon as a single parent. Kate and her son made their home aboard a houseboat moored on an English canal.
Because the cult based at New World University believed she might be able to lead them to the Brigadier, who they thought possessed a locus vital to the Great Intelligence, she and Gordon were harassed by students who staked out her houseboat. Frightened, she reconnected with her father and briefly fought alongside him and Sarah Jane Smith against the New World group. After the Great Intelligence was again defeated, she entered into a friendlier relationship with her dad, and ensured that Alistair could have a relationship with the grandson whom he had not previously known existed.
In 1997, Alexander Christian used Kate to get in touch with her father. He wasn’t allowed to know the Brigadier’s number for reasons of security, but he was readily told Kate’s (it was 0122 69046).
In October 2003, Kate responded to a message from ex-UNIT operative Douglas Cavendish to investigate a haunting. Arriving at Cavendish’s isolated cottage, she faced Mastho, one of the last Daemons — a demonic alien race which her father had previously battled more than thirty years before. They discovered that a future cult called the Sodality had revived, and transported Mastho through time for their purposes. This meant that the threat to the present was neutralized.
Kate later married, had a second child, and was divorced. (Death in Heaven) She dropped the name “Lethbridge” when she joined UNIT so as to be judged on her own merits and not be perceived as being the beneficiary of nepotism. (The Power of Three)
Kate bids the Eleventh Doctor farewell following The Year of the Slow Invasion. (The Power of Three)
Her father nevertheless quietly mentored her until his death. He instilled in her the position that “science leads”, something he learned from “an old friend” (i.e., the Doctor). Kate eventually rose to the post of Head of Scientific Research, and established UNIT as a military organisation led by its scientists. In this capacity, she summoned the Eleventh Doctor and Amy Pond via his psychic paper to the Tower of London to investigate the mystery of the Shakri cubes. Kate attempted to warn the governments of the world of the cubes when they started their countdown. (The Power of Three)
She ordered the airlifting of the TARDIS to London, to the agitation of the Eleventh Doctor. Kate summoned him there under the royal orders of Elizabeth I, giving him a sealed letter from the Queen and proving her credentials by showing him the impossible painting Gallifrey Falls No More within the National Gallery.
The letter decreed that the Doctor be made curator of the Under-Gallery, and that he be summoned should there be any disturbance there. Kate led the Doctor and Clara Oswald to the scene of the disturbance. There, she showed them more paintings, all landscapes with glass on the floor next to them. When the Doctor jumped through the fissure and met the Tenth Doctor and the War Doctor. Clara attempted to follow him before Kate restrained her.
As she left the room to request important UNIT files, Kate was accosted by a Zygon who had escaped through one of the paintings and then took her image and pretended to be the real Kate to Clara Oswald. Osgood saved her from the Zygon nest. Kate, along with McGillop and Osgood, confronted their Zygon doppelgängers in the Black Archive. Kate ordered the countdown to detonate the Archive which would not only have prevented the Zygons from having control of the many weapons stored there, but would also destroy London in the process. The Zygon used Kate’s voice to stop the detonation, but Kate countermanded the order.
Kate and her Zygon counterpart bickered over the detonation, before the Eleventh and Tenth Doctors triggered the memory modifiers to confuse everybody in the room as to whether they were a Zygon or a human. The two Kates stopped the countdown, and negotiated a peace treaty. (The Day of the Doctor)
In the same year, 2013, UNIT lost one of their lead scientists, John Foster, who died in a car accident. Kate and the team spent the next year trying to reactivate the projects (based off of the technology left by the Cybermen in 2007), but found various codes that blocked access. She visited the home of Foster’s family, where his family was having trouble coping with the accident. It was then that another John Foster broke through into their dimension to meet with his daughter, unbeknownst to them. When London was attacked by a species originating from the void known as The Fractures hunting foster, Kate aided the Twelfth Doctor and Clara Oswald in pushing back against the aliens.
When the Cybermen emerged from St Paul’s Cathedral in the 21st century, Kate arrived with Osgood and UNIT and took Missy and the Twelfth Doctor into custody. She told the Doctor that he was President of Earth and worked with him to battle the Cybermen. When the Cybermen attacked the plane Boat One, Kate was blown out by Missy. She was saved by her father, who had been converted into a Cyberman, and returned safely to the ground. (Death in Heaven)
When Missy returned and froze all the planes on Earth, Clara came to UNIT and Kate helped her contact Missy. (The Magician’s Apprentice)
Kate and UNIT went to investigate a Zygon invasion that broke out after a rogue gang of them decided a peace treaty with humans was unacceptable. She was determined to bomb the rebel Zygon settlement, but the Doctor convinced her that this would only make the situation worse – threatening to radicalize every peaceful Zygon on the planet.
Kate followed a lead to Truth or Consequences in New Mexico to try and find the root of the uprising. Here she encountered the one remaining inhabitant of the town, police sheriff Norlander, who explained how the trouble started – the accidental revelation of a young Zygon in true form who “hadn’t learnt to preserve his body print” had caused panic, distrust and violence within the human population. As the truth of what had really occurred began to dawn on Kate, she realized that Norlander was in fact a Zygon rebel agent. (The Zygon Invasion)
She was then attacked but managed to kill the Zygon, before taking its place to fool the High Command. Kate was present in the Black Archive when the Twelfth Doctor forced her to become an ambassador for the human race in an attempt to negotiate a new peace treaty alongside the Zygon High Command leader Bonnie in which a standoff occurred involving the Osgood Box. However, her memories of the Box were soon erased. (The Zygon Inversion)