It's tough to pick out just one great David Tennant moment from the Series 3 episode Utopia. There are just so many! One close contender for best moment has to be when the Doctor and Martha are talking in the TARDIS, having just arrived at the end of the universe. The Doctor says they shouldn't be there, they should really go, but there's a long moment where they look at each other... and then the Doctor grins, and the audience knows that he's not going anywhere! This moment underlines an important point about the Doctor's character. He's not going to turn back on exploring somewhere, even when it's against all the rules. But an even more profound moment for his character comes towards the end of the episode, as he and Jack are making the final preparations for the launch of the humans' rocket. Martha has discovered Professor Yana has a fob watch just like the Doctor's one, and she comes racing down the corridor to tell the Doctor. After hearing the news, Jack reasonably points out that Yana could well be a Time Lord and Martha says, "That's brilliant, isn't it?". The Doctor replies calmly at first, saying that yes, it could be, but it depends which one Yana is. But as he thinks more about it, he becomes increasingly panicked, asking first, in a relatively calm manner, "What did he say, Martha?" But in a split second, before Martha can even answer, he loses his temper completely, screeching "What did he say!?" It's one of the very few times the Doctor utterly loses his cool with a companion and Tennant plays the moment perfectly.