Time Crash is a mini-episode of Doctor Who, broadcast as part of the Children in Need special in 2007. It starred David Tennant as the Tenth Doctor and Peter Davison as the Fifth Doctor. The action is set immediately between Last of the Time Lords and Voyage of the Damned.
The episode has the TARDIS caught in a temporal implosion, which The Tenth Doctor and the audience know is caused by colliding with the older TARDIS of the Fifth Doctor. The temporal effects have apparently temporarily aged the Fifth Doctor some years, which is a great way to explain why Peter Davison looks older than he was. The rest of the episode is the Fifth Doctor realizing just who he is face to face with, and then the two of them discussing their voyages. It ends with a speech that the Tenth Doctor, or perhaps David Tennant, gives to the Fifth Doctor, or perhaps Peter Davison, telling him how he formed his persona.
David Tennant, barely out of his toddler years, was inspired to take up acting by watching Doctor Who. Maybe the greatest thing about Tenannt's tenure was how much sheer fun he exhibited at getting to play his dream role. In the "Time Crash" mini episode, we get to see not just the Tenth Doctor, but we get to see Tennant himself, channeling every fan who has ever dreamed of getting to play the Doctor. And the story goes beyond fiction: the man he is talking to is soon his Father-in-Law. The whole thing is great fun, and it makes the viewer feel part of the great Doctor Who family. It is also the only time in the revival of Doctor Who that we have seen a new Doctor and an old Doctor meet. For these reasons, although it is a short episode made more for novelty than anything else, it is still a great thing for a fan of Doctor Who to watch.