The Infinite Quest is an animated story of Doctor Who, featuring the voice acting of David Tennant as The Tenth Doctor and Freema Agyeman as Martha Jones. It was originally shown as a series of 3 minute episodes on children's show Totally Doctor Who, before being compiled into a single 45 minute long episode that was broadcast the same day as the Series 3 finale.
The name of the story comes from a search for the lost starship The Infinite, which can fulfill wishes. The Spaceship is sought after by space tyrant Baltazar, so of course The Doctor and Martha have to get there first. The spaceship can be found by following a trail of datachips, which lead the Doctor and Martha to a series of worlds, which in true Doctor Who fashion, have their own complications and political intrigue. When the story is finally concluded, there is a twist and a lesson, the villain is defeated, and The Doctor and Martha move on.
Considering that this was an animated story, and was broadcast in 3 minute chunks on a children's show, it is a pretty respectable Doctor Who story. Although it is a fast paced adventure story, with travel to a variety of exotic locations (a formula first used in Season 1's The Keys of Marinus), it still has the complex twists we expect from Doctor Who. It even makes a few political and social points.
The animated format allows for some scenes and characters that would be very difficult in a filmed episode. The Tenth Doctor's mannerisms, which can be somewhat hammy, lend themselves to animation, although Martha Jones' seems to be much less natural in this format. Overall, however, I think that this episode used the animation format very well to tell a good Doctor Who story.