Doctor Who: Season 3, Episode 8

3 - 8: Human Nature

To evade alien mercenaries seeking his regenerative ability, the Doctor transforms himself into a human being with no memory of being a Time Lord, then hides as a schoolteacher in the early 20th century while Martha stays nearby as a maid to monitor him. But the mercenaries arrive, while a schoolboy gets hold of the watch containing the Doctor's "essence."