The David Tennant Treat 4 Today is the really amazing Time Crash video.
This was first broadcast on 16th November 2007 to raise money for Children In Need. It was written by Stephen Moffat and slotted in perfectly between the end of Series Three and the start of Voyage Of The Damned.
It is a hilarious 8 minute episode which shows David Tennant as the Tenth Doctor meeting the Fifth Doctor played by Peter Davison.
It was filmed all in one day (7th October) with everybody involved donating their time to raise money for Children In Need.
Viewing figures peaked on Children In Need night between 8.15pm and 8.30pm when Time Crash aired with a total of 11.0 million viewers. The episode was therefore the most-viewed since the show's revival in 2005 for a few months. This rating was later beaten by Voyage of the Damned which received 13.3 million viewers. (Donations to the charity also peaked during Time Crash's airing. )
As well as some great writing the acting of the two actors ensured that this episode is one of the most loved by Doctor Who fans.