9. The Doctor Pretends To Be John Smith In The Family's Spaceship
The two-part story Human Nature/The Family of Blood in Series 3 provides Tennant with myriad opportunities to shine. He plays the human John Smith so differently from the Time Lord that it's easy to accept that they are two completely separate people. Even more impressively, there's a moment when Tennant seamlessly transitions from Smith to the Doctor before the viewers' eyes, showing just how versatile he is. It happens when Smith enters the Family's spaceship to offer them the Doctor's watch, and all the secrets it contains, in exchange for the lives of those he loves. The possessed Baines takes the watch and shoves Smith into a control panel. As Smith cowers on the floor, Baines opens the watch... only to find that it is empty. And Smith is revealed to be the Doctor merely pretending to be human with a simple "olfactory misdirection". The Doctor might be an alien but he understands humans well enough to impersonate one. What happens next is chilling. It's not the part where the Family's ship explodes due to the Doctor's sabotage. It's his subsequent merciless revenge on each Family member - the unrestrained "fury of the Time Lord" - that makes viewers' blood run cold. To remind everyone just how scary the Doctor can be when there isn't a companion around to serve as a moderating influence, here's a video showing the entire sequence beginning with "Smith" entering the Family' ship.