The Monk

Mortimus, later known as The Monk, was a former friend of the Doctor and a renegade Time Lord.

Youth
At the Academy, Mortimus was a friend of the Doctor's, and a member of a cabal of ten rebellious young Gallifreyans at the Academy known as the Deca. (PDA: Divided Loyalties)

Reunion with the Doctor

 * See the Monk's TARDIS.

Mortimus was the owner of a Mark IV TARDIS and said that he had left the Doctor's home planet some 50 years after the Doctor. (DW:The Time Meddler). He decided that he liked to meddle with history, specifically the history of Earth, and to change it for his own amusement and for what he considered to be the better. He lent mechanical assistance to the builders of Stonehenge and gave Leonardo da Vinci tips on aircraft design. At the time that the Doctor, encountered him, the Monk attempted to prevent the Norman Conquest of 1066 as part of a plan to guide England into an age of technological prosperity. On that occasion he wore the guise of a monk in order to gain the trust of the locals, hence the name by which he is most often referred. The Monk was at this time a well-meaning but childish man who was not half as clever as he thought he was, and who never seemed to realise the seriousness of what he was doing. The Doctor sabotaged the dimensional circuit of Mortimus' TARDIS, making it the same size inside as outside, reducing the interior to dollhouse proportions.(DW: The Time Meddler)

Second Encounter with The Doctor
The Doctor encountered him again on the volcanic planet Tigus, where Mortimus sabotaged the lock on The Doctor's TARDIS, though that did not stop him from getting inside. The Doctor stole Mortimus' direction controls in order that he could use it himself as part of his effort to stop the Daleks. Mortimus' TARDIS landed in ancient Egypt. Having heard of the Daleks, the Monk went along with their plans, in order to avoid being exterminated himself, while trying without success to convince the Doctor and his companions of his honourable nature. While there, the Doctor tinkered with the chameleon circuit from Mortimus' TARDIS, making it assume various shapes, finally that of a police box. He stole its partially compatible directional unit, leaving Mortimus stranded in a cold, icy location planet. (DW: The Daleks' Master Plan)

Ally of the Ice Warriors
Mortimus then teamed up with the Ice Warriors and battled the Doctor in a complex scheme involving alternative Earths and a plan to build a giant sonic weapon. Mortimus now preferred his own TARDIS to appear as a police box, did not wear a monk's habit, and referred to himself as the Time Meddler. (DWM: 4-Dimensional Vistas)

The Sleeze Brothers
At some point in the 21st century, the Monk tried to rig elections in what might have been the United States to stop President Sintatra from winning a third term of office. As he began this mission, he landed his TARDIS on a busy freeway. The Sleeze Brothers, El Ape and Deadbeat, collided into it, causing significant damage to their vehicle. At the same time, the companion-less Doctor landed his TARDIS in the same area.

Besieged by both angry brothers and an irate Doctor, the Monk slipped back into his TARDIS and took off. The Brothers then hijacked the Doctor's TARDIS at gunpoint and ordered him to follow the Monk's TARDIS through time. A chase ensued, and the two TARDISes flitted through time to several famous mysteries in Earth's history. Eventually, the Doctor and the Sleeze Brothers caused the Monk's TARDIS to implode, which apparently caused the creation of the Bermuda Triangle. How, when, or if the Monk reconstituted his TARDIS wasn't clear. (DWM: Follow that TARDIS!)

Death's Champion

 * At some point, the Monk regenerated at least once. Therefore, when they next met, the Doctor did not recognize him by sight.

Mortimus then created a series of alternate timelines (NA: Blood Heat, The Dimension Riders, The Left-Handed Hummingbird, Conundrum) in a scheme employing the Chronovore Artemis. Mortimus now used his real name and posed as a 1976 record executive in England. He had also, at this time, served as the Champion of the Eternal Death in the same way that the Doctor served as Champion of Time. In so doing, he had made himself servant to a being much more powerful and intelligent than himself. Mortimus aided the Vardans' scheme to avenge themselves on the Doctor and the Sontarans by conquering Earth, a planet of continued strategic value to the Sontarans and of importance to the Doctor. His plan was undone thanks to the Doctor's companion Ace, who pretended to side with him until she could free Artemis. The vengeful Artemis subsequently took Mortimus away to make him pay for her imprisonment. (NA: No Future)

Mortimus later tried to take Antonio Salieri back in time to kill Mozart when he was still a child. Mortimus was stopped by the Doctor and the Tramp, Mozart escaped unharmed. (ST: The Tramp's Story)

The Book of Kells
Mortimus somehow managed to escape Artemis and, gaining Lucie Miller as a Companion, he travelled to Ireland in 1006, seeking an artefact known as the Book of Kells. During this time, he once again encountered the Doctor. (BFA: The Book of Kells)

After an arguement with Lucie, the Monk abandoned her on Deimos. The Monk then tampered with Ice Warrior history to create a new timeline in which a group of Ice Warriors woke up early and re-terraformed Mars to take it back from the humans. This plan was stopped by the Doctor, who set history on its proper course. The Monk managed to persuade the Doctor's companion, Tamsin Drew, to leave the Doctor and begin travelling with him. The duo then set off to find the Daleks' help in defeating the Doctor. (BFA: The Resurrection of Mars)

Behind the scenes

 * The Monk has the distinction of being the first Gallifreyan other than the Doctor and Susan to be seen in the series. (During The Daleks' Master Plan, the Daleks actually seem to refer to the Doctor and the Monk as "humans", though they may simply use the term loosely). The name Time Lord would not appear until The War Games, however.
 * For some time there was speculation whether the Monk was actually an earlier regeneration of the Master or the War Chief, who appeared in The War Games. This is stated as fact in The Doctor Who Role Playing Game, which tended to create its own continuity. However, this theory has been contradicted by the Monk's appearances in novels and comics. Though this is rendered somewhat moot due to all spinoff material being of uncertain canoncity. Also, it would be possible that the Doctor is simply meeting the Master in his past.