Exile on Earth

Following the defeat of the War Lord's allies, a triad of three Time Lords tried the Doctor for interfering in the affairs of the universe, The Doctor' sentence had two conditions:


 * The Doctor's regeneration into a new incarnation, as too many people recognize him there.


 * Exile to Earth in the 20th century. (DW: The War Games)


 * We do not actually see the Second Doctor's regeneration.

The regenerated Doctor's TARDIS re-materialized in the Oxley Woods at the same time as an invasion by the Nestene Consciousness, in the late 20th century. An adventure followed, in which the Doctor helped UNIT defeat the Nestene Consciousness's Autons. The Doctor would stay on as UNIT's unpaid scientific advisor in exchange for a place to park the TARDIS and technical resources he needed to his ship. (DW:Spearhead from Space).

Bitterly resentful of his punishment, the Doctor suffered partial memory loss as a condition of his time travel, specifically as concerned the secrets of time travel (DW: The Claws of Axos.). He also did not remember all the materialization codes he needed to operate the TARDIS (DW: The Three Doctors).

While based on 20th century Earth, a Time Lord appeared to warn the Doctor of the coming of the Master, an indirect way of sending him on a mission to defeat the Master's plots. (DW: Terror of the Autons).

The Time Lords allowed the Doctor to full use of his TARDIS to stop the Master, again, this time on an Earth colony in the future (DW: Colony in Space). Later, they allowed him to travel to Peladon for unspecified reasons (DW: The Curse of Peladon). Once, they even sent him on a direct mission to convey a message to Ky, a Solonian (DW: The Mutants).


 * An inconsistency between the Time Lord's doctrine of non-interference and their sending the Doctor on mission has a possible explanation. Very possibly, the Doctor's missions during this period of his life, occured on behest of the Celestial Intervention Agency. As may the Doctor's "coincidental" placement where and when they piloted his TARDIS.::

Without explanation, the TARDIS also travels through time to Atlantis to catch the Master without any explanation of it can do so, given the Doctor's exile. (DW: The Time Monster)


 * Perhaps the Time Lords had a hand here, too?::

Finally, as a reward for defeating Omega, the Time Lords rescinded the Doctor's exile on Earth, granting him once more the freedom of space-time travel. (DW: The Three Doctors)