Third Doctor

Biography
The Third Doctor was the result of a forced regeneration imposed upon the Second Doctor by the Time Lords. This Doctor was exiled to 20th century Earth and his memory blocked so he could not use his TARDIS to escape. He struck up an arrangement with old friend Brigadier Lethbridge-Stewart wherein he would serve as UNIT's Scientific Advisor, and in return could use UNIT's technical and scientific facilities to attempt to make the TARDIS operational again.

After the arrival of the Master on Earth, the Time Lords began allowing the Doctor limited use of his TARDIS, but always compelling it to return him to 20th century Earth when his missions were completed, essentially turning him into "some sort of a galactic yo-yo ("The Claws of Axos")." When the first three Doctors successfulyy stopped Omega in his quest to destroy the Time Lords, they rewarded the Third Doctor by restoring his knowledge of the TARDIS codes and lifting his exile ("The Three Doctors").

The Third Doctor continued to assist UNIT, though his presence on Earth was now much more intermittent. Finally, to defeat the Spiders of Metebelis III, he sacrificed his own life by exposing himself to lethal levels of radiation. With the assistance of his old mentor K'anpo Rampoche, he was able to regenerate into the Fourth Doctor.

Characteristics
The Third Doctor was heavily involved with UNIT while stuck on Earth, though he tended to have problems with the Brigadier's destructive manner. The Third Doctor had numerous encounters with the Master, and was the first known incarnation to encounter the Silurians/Sea Devils, Ogrons, and the Autons.

Habits and Quirks
The Third Doctor was known for his love of gadgets and a passion for his vintage car Bessie. He was the most action oriented, using a wide variety of martial arts including Venusian Akido.