Master of the Land

The Master of the Land, also known as the Master of the Land of Fiction, the Master, and the Controller, was the title given to the ruler of the pocket universe known as the Land of Fiction. He or she was, in turn, connected by the computer known as the Master Brain. Presumably, the Master Brain wanted to harness a human's powers of imagination, as the first Master noted later, the computer needed imaginative minds. (TV: The Mind Robber) As Master one could create fictional characters into being, control the clockwork soldiers and turn people from the real world into fiction, but not directly.

Unnamed English writer
The first Master encountered by the Doctor was originally employed in writing the adventures of Captain Jack Harkaway for a boy's magazine called The Ensign. He wrote five thousand words each week for twenty-five years. In the summer of 1926, when he must have fallen asleep writing, the Master Brain found him and connected him to the Land of Fiction, where he became the ruler. As the Master would note later, the computer needed imaginative minds. The Master Brain enslaved the Master and intended to add the Second Doctor to the system so it could invade Earth and all humans would be adjusted to the Land, leaving Earth to be taken over.

The Doctor, Zoe, and Jamie arrived in the Land by accident. The Master planned to have the Doctor take his place. Using his power, he put the trio through tests to find if the Doctor could do it. Zoe and Jamie overloaded the Master Brain and the Doctor saved the Master from its destruction. Once the Master was disconnected, the Land dissolved. The Master was presumably returned to his proper place in time and space: England of 1926. (TV: The Mind Robber)

Zoe Heriot
During the Cybermen invasion of the Land of Fiction, the Land of Fiction had a new master, but was this time was a Mistress. She was using the clockwork soldiers, and fictional characters such as Alice and Dracula to fight a failing war against the Cybermen, who had partly if not totally cyber-converted creatures and beings of the Land, such as giants, ogres, werewolves, Valkyries and fairies.

The Mistress of the Land of Fiction was the real-life, older Zoe, who had found out she had mysteriously aged two years and was captured in a Cyberman attack on the Wheel. When Zoe's memories were triggered, she sent the Cybermen into the Land of Fiction and brought the Sixth Doctor there to defeat them, creating Jamie as a friend and as a mystery to solve. After a Datamat, which looked similar to a Cybermat, started climbing up Jamie's kilt, the Zoes were told to work on writing the fictional Cybermen, who had been converted from fantasy creatures, out of existence. Captain Nemo, finding himself with a cyber-converted Dracula, was rescued by the soon-to-be-killed Karkus, sent by Zoe. The Doctor used the Datamat to erase the fictional Cybermen from existence. The fictional Zoe was killed. Alice was elected to be in charge of the Land of Fiction. (AUDIO: Legend of the Cybermen)

Jason
Years later, as part of his elaborate scheme to gain revenge on the Doctor, Mortimus installed as master Jason, a mentally unstable 17 year old boy from 1993 Earth. Jason created a fictionalised version of the Doctor called "Dr. Who".

Jason constructed a town called Arandale, inhabited by various fictional characters such as the White Knight and Doctor Nemesis. He trapped the Seventh Doctor, Ace, and Benny in a scenario where the power source that gave the White Knight his powers would soon explode and destroy the town unless the Doctor either went along with the story and thus became trapped in the Land for good, or took the Master's place. By tricking Jason into introducing highly disruptive McAllerson's Radiation into the Land by claiming that it could be the source of the White Knight's powers, the Doctor escaped after the Radiation release damaged the Land long enough for him and his companions to return to the TARDIS. (PROSE: Conundrum)

After defeating the Monk, the Seventh Doctor contacted the Time Lords, who returned Jason to his native time of 1993. (PROSE: Head Games)

During Jason's time as Master of the Land, he stated that the Doctor was "more than qualified" to receive the title. The ability to manipulate plot and overwrite story was partially wielded by the Doctor, including rewriting the story to gain information, summoning characters from other fiction, and bending reality with a thought. The Doctor used these abilities to battle against Jason, giving the TARDIS crew a chance to escape from the Land of Fiction. (PROSE: Conundrum)

L. Frank Baum and Dorothy
tricked L. Frank Baum into becoming the Master of the Land of Fiction, telling him he was in a theme park based on his world of Oz. Although Baum was the one actually connected to the computer, Missy considered herself the real Mistress of the Land and Baum an "under-master". Missy brought the Thirteenth Doctor, Yaz, Graham, Ryan and their temporary companion Theodore to the Land of Fiction in an attempt to drain the Doctor's courage, heart and brains. The Doctor foiled Missy's plan and forced her to disconnect Baum. However, without him the inhabitants of Oz would be terrorised by the fictional villains displaced by Oz. Theodore assumed the title ("Queen of the Land of Fiction") to keep Oz in place, and was able to physically become her inner female-identifying self, renaming herself Dorothy. Dorothy remained to keep Oz intact and add to it with her knowledge of books, with K9 Mark V remaining as company. (PROSE: The Wonderful Doctor of Oz)