Doctor Who: The Timey-Wimiest Episodes Not By Steven Moffat
3. Utopia / The Sound Of The Drums / Last Of The Time Lords (2007)
Series three of the revived Doctor Who concluded with an epic three-part story that featured the return of the Master, in one of the more complex time-travel dramas that the series had offered to that point.
The story kicks off in the far future, where the Tenth Doctor encounters his old enemy, the Master, in the guise of the kindly Professor Yana. Once exposed, the Master steals the Doctor’s TARDIS, and goes back in time to become the Prime Minister of England, a character who had been frequently referenced but remained unseen all through the series.
The Master’s plan turns out to be to trick the future survivors of the human race into becoming monsters, coming back in time, and wiping out their ancestors, with the obvious paradox being held in place by a special machine created out of the Doctor’s TARDIS.
At the story’s conclusion, that “Paradox Machine” is deactivated, and the previous year’s devastation is undone.