Kate Stewart Kate Lethbridge - Stewart ( known as Kate Stewart after joining UNIT 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 - heartedly 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 investigating UNIT and 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 organisation , 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 peace treaty between human ity and Zygon s on Earth . Biography . Childhood . 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 . ( HOMEVID : " Downtime " ) At the time of the Wenley Moor Silurian incident , Kate was five . It was during this mission that Alistair realised 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 . ( PROSE : " The Scales of Injustice " ) John Benton met Kate as a child , and called her " little Katie " . ( AUDIO : " Call to Arms " ) It may have been around this time that Kate spent some time with her father at Ealsdon House in Scotland . She became friends with Ben Donelly for a while , the two of them would regularly run up and down the long gallery singing a nursery rhyme about a ghost that haunted the place called the Grey Man . ( AUDIO : " Invocation " ) Later life . Eventually , Kate grew estranged from her father . ( HOMEVID : " Downtime " ) 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 . ( PROSE : " Downtime " ) 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 . ( HOMEVID : " Downtime " ) 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 ) . ( PROSE : " The Dying Days " ) 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 Daemon s — 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 neutralised . ( HOMEVID : " Dæmos Rising " ) Kate later married , had a second child , and was divorce d . ( TV : " 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 . ( TV : " The Power of Three " ) Meeting the Doctor . 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 cube s . Kate attempted to warn the governments of the world of the cubes when they started their countdown . ( TV : " The Power of Three " ) Stopping a Nestene invasion . She invited Osgood to Simon Devlin ' s demonstration of a 3D printer . There she met with Jacqui McGee who knew who she was . She was concerned that the technology could be used for war applications . Osgood ( AUDIO : " Vanguard " ) She sent Sam Bishop to the Gobi Desert with Osgood to search for a Swarm Leader . She then met with Devlin to find out more information about him to see if he was connected with the Nestene Consciousness . ( AUDIO : " Earthfall " ) She asked Osgood to examine the energy unit and the 3d printer . She was kidnapped by Devlin . Devlin wanted to convert her into an auton ( AUDIO : " Bridgehead " ) Bishop sent her to Black Archive 5 to discover an item the Ninth Doctor used against the Nestene , anti - plastic . She then went to Devlin Futuretech to buy some time for Osgood and to learn more about the Nestene ' s plan . ( AUDIO : " Armageddon " ) Trouble at the National Gallery . UNIT had in its possession a picture of Kate with River Song . ( TV : " The Day of the Doctor " ) 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 with a nuclear warhead 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 War , Tenth and Eleventh 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 . ( TV : " The Day of the Doctor " ) Tengobushi incident . She got angry when she discovered that Lyme Industries had took some of UNIT ' s property under Peter Latcham ' s orders . She tried to get it back in a covert mission but she had discovered that Lyme industries had taken it all to their headquarters . ( AUDIO : " Power Cell " ) She went to Geneva to get head office to authorise and investigation into the leak in the UK Government and the Tengobushi attack . She got annoyed when Grant Avery called the UK Government to inform them of Kate ' s actions . After an attack happened on a French mountain near Geneva she helped Avery to investigate . Discovering that it was similar to the attack in London she managed to persuade Avery that she was right . When Avery was captured by the Tengabushi she devised a way for her and Osgood to get inside the ship to aid his escape . She then went back to London to stop Dokan getting into the Tower of London to get the Kamishi energy cell . ( AUDIO : " Death in Geneva " ) She and Osgood had to fly the plane they were taken after the Tengobushi had killed their pilots . She asked Josh to kidnap Lyme in order to get information on the artefact the company had . She met with Dokan to find out why he wanted the energy cell . She tricked Dokan into meeting her in the Bloody tower and then let a bomb off to destroy the Kamishi energy barrier . ( AUDIO : " The Battle of the Tower " ) She was placed under arrest by Latcham , but was later freed by Osgood and Carter . With the experimental aircraft at the base she went to rendezvous with Bishop in South America , and then travel to Antarctica to undercover Lyme ' s generator there . She confronted Lyme and told her the consequences of using alien technology . When the Kamishi arrived to taken their technology back , she managed to persuade them not to kill everyone who had knowledge of their technology but use their mind wipe chemicals instead . Back in London she took delight in personally removing Latchen ' s memory . ( AUDIO : " Ice Station Alpha " ) Investigating the Silence . She decided to up her training of shooting and asked Carter to help her . She put Shindi on undercover observations at Helena Faversham ' s place to undercover the strange energy readings Osgood was detecting . She was concerned about the conspiracy theorist . She posed as part of the charity commission to talk to Faversham . She tried to negotiate with the Silent s but they wouldn ' t do it . ( AUDIO : " House of Silents " ) She forgot the whole incident with the Silents , though could remember going to Faversham ' s house . Jacqui got her to remember the investigation and with Osgood and Bishop ' s help they found the evidence . She found out that the Silents were influencing the UK population to trust Kenneth LeBlanc and that Carter had been brain washed too . ( AUDIO : " Square One " ) She found it disconcerting about Carter ' s involvement in the resignation of the PM and that she had to remind her troops about the Silents . When Bishop showed him the moon landing footage with the Silents in them , she realised that it must have been the Doctor who put it there . Kate became suspicious when the Silents got Carter to order a high amount of explosives . She thought that they were planning to assassinate both LeBlanc and the out going prime minister , as she thought that this was the best way to get media attention and overrule the Doctor ' s message to kill them all . ( AUDIO : " Silent Majority " ) She was concerned after they lost contact with Osgood and Bishop on the UNIT satelite . She was trying to investigate the cause of the loss of communications . She discovered that the Silents were manipulating the powers of the world to the brink of starting the war with each other . After she realised the the Silence were also provoking the Telokni to fire on the UNIT space station , she used the footage of the destruction cut with the Silents saying they were leaving the Earth , using McGee ' s editing skills to do it . ( AUDIO : " In Memory Alone " ) The Silurian invasion . Kate and Osgood investigated a Silurian nest at Scafell when they started to attack . She and Osgood escaped and made their way to John Benton ' s pub . Whilst there she was aided by Benton and Mike Yates , to help defend herself against the Silurians . She couldn ' t leave Benton behind when he became injured and stayed when Osgood and Yates left in Bessie . After the base was destroyed she asked them to come to London for a debrief . ( AUDIO : " Call to Arms " ) Kate later went to find Jo Jones for a mission , and discovered her chained to a factory protesting the pharmaceutical industry . After ' Project Charybdis ' went off course she immediately went to help them . She almost destroyed the Sea Devil base , but Osgood stopped her . ( AUDIO : " Tidal Wave " ) Osgood and Kate made their way to a Silurian nest in Greece to find a control device for the Hybrids that Anthony Burmaster had released . She made her way through the base but was attacked by Tryska and poisoned . Fortunately she was rescued by Bishop and given a broad spectrum anti - venom . Whilst in the bases computer room she managed to find information on the variation of the Silurian and Sea Devil species and where she could find some anti - venom . ( AUDIO : " Retrieval " ) She was still recovering from the poison by the time she managed to get back to UK . Kate passed local control of UNIT in the UK to Mike , Benton and Jo due to their experience with Silurians and Sea Devils before , whilst she couldn ' t get back to base . Kate sent Sam to a weather station to brew up a storm off the coast of UK . This caused the Silurians to slow down and neutralised their forces . She tried to negotiate their surrender by allowing them to go back into hibernation . She later put Jastrok and Kalana into hibernation under the Black Archive so that their people could punish them . ( AUDIO : " United " ) Brief Encounters . Kate and her team flew to South America on intelligence that a local resistance force had obtained alien technology . There they met resistance leader Captain Gonsalves and discovered she had a captured Dalek in her secure bunker . Kate was able to purchase the main body of the Dalek , but not the armaments which she originally believed was part of the deal . When the Dalek reveals there are others of its kind in the surrounding jungle , Kate and Shindi take the Dalek creature to find the other Daleks , but the creature escapes . Returning empty handed Kate was taken prisoner and , together with Osgood , discovers Gonsalves is part of a worldwide auction house of alien technology and weapons . Having re - entered its casing the Dalek kills Gonsalves and self - destructs , destroying the bunker . Having escaped the explosion Kate and her team were then rescued by the American arm of UNIT . ( AUDIO : " The Dalek Transaction " ) Noticing an irregularity in UNIT ’ s paperwork Kate headed to Ealdon House in Scotland , a research facility set up in her father ’ s time but abandoned soon after . There she met Ben Donelly , the caretaker , and his mother Alice . Kate ’ s memories of visiting the house soon returned to her and the unsettling nature of the long gallery . Entering there again she had another encounter with the supposed Grey Man and was nearly frightened out of her wits . After recovering Kate was joined by Osgood and Josh and discovers Alice had discovered a particular resonance in the house that induced fear in the humans . She had wanted to harness this resonance as a weapon before Kate ’ s father stopped her . With an unknown alien presence drawing near , responding to a message Alice left years ago and hearing of mass outbreaks of terror in the capitol , Kate and Osgood deciphered and completed the message , obverting the crisis . ( AUDIO : " Invocation " ) Kate observed Shindi ’ s arrival in the Ardennes , via the hologram projector , and his discovery of a Sontaran capsule . Suddenly they are ambushed by a squad of Sontarans and Kate looses contact with Shindi . Later when he and the Sontarans have returned to England Kate and some UNIT troops meet up with them outside the Torrence Research Foundation having tracked them using the hologram projector . Allying themselves with the Sontarans UNIT storm the building and find Osgood held captive . They also uncover a group of Sontarans cloned by Professor John Torrence hoping to sell them off as warriors for hire in the worldwide auction . While shutting the operation down Kate learned from Christine Colley about the mysterious head of the auction , the Overseer . ( AUDIO : " The Sontaran Project " ) Kate was worried when an experimental travel capsule , obtained from the auctioneers , suddenly vanished from Osgood ’ s lab taking her and Josh with it . On their return Kate was bemused by how familiar Josh and Osgood had become with each other . She soon realised this Osgood was in fact from a sideways dimension where UNIT was run by a fascist state . With the other Osgood on the run , while armed and dangerous , and the real Osgood returned Kate was the only one to realise this , preventing Shindi from taking the real Osgood prisoner . Soon the other Osgood returned and attempted to escape again in the capsule . Kate and her team were saved when the capsule just managed to disappear before the dimensional gateway imploded . Kate then informed her team Sam had more information on the auctioneers and they were to report to her office . ( AUDIO : " False Negative " ) Actions with the Twelfth Doctor . In 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 Fracture s hunting Foster , Kate aided the Twelfth Doctor and Clara Oswald in pushing back against the aliens . ( COMIC : " The Fractures " ) When the Cybermen emerged from St Paul ' s Cathedral in the 21st century , Kate arrived with Osgood and UNIT and took 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 under Missy ' s command , part of the fuselage blew off and Kate fell out . She was saved by her father , who had been converted into a Cyberman , and returned safely to the ground . ( TV : " Death in Heaven " ) When Missy returned and froze all the planes on Earth , Clara came to UNIT and Kate helped her contact Missy . ( TV : " The Magician ' s Apprentice " ) Kate and UNIT went to investigate a Zygon invasion that broke out after a splinter group 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 radicalise 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 C . 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 realised that Norlander was in fact a Zygon rebel agent . ( TV : " 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 . ( TV : " The Zygon Inversion " )