Although a theme that had been explored before, with the Meddling Monk ("The Time Meddler") and the War Chief ("The War Games"), the introduction of the Master as the main villain for every story of series 8 saw a true Evil Counterpart for the Doctor -- a Moriarty to the Doctor's Sherlock Holmes, as the BBC Classics website puts it. Eventually, though, the Doctor persuades the Master that the Nestene Consciousness will have no use for him once they arrive, and they work together to repel the Consciousness into space. Watch it here