Y'know, yet another way that this episode is quite special that it actually brings the time travel element of the show to the forefront. In most episodes, the TARDIS is just a convenient gateway to adventure, and once the doctor's where the writer wants him to be, the fact that he's got a universe-spanning time machine is an obstacle to suspension of disbelief that you gotta overlook.
![Devious :] :]](http://www.enworld.org/forum/images/smilies/devious.png)