This is an animated episode featuring the Tenth Doctor and Martha Jones, battling the space villain Baltazar (voiced by Anthony Head), who tricks them into going on a quest for "The Infinite", a mysterious object from the dark times. (How careless of the ancients to leave that stuff lying around!) The plot takes our heroes to various planets where the find the clue that lead them to the lost object. The animation is OK - nothing for Studio Ghibli to worry about, but acceptable. The story is fairly simple - aimed at a somewhat younger audience than the regular show. There's not alot of snappy quotes, although I was fond of "He promised me all the gold I could eat", and was amused that the Doctor, when brought before a prison warden, was revealed to have 3,005 outstanding convictions! Overall, it's a quick, amusing thing - a sort of "Doctor Who lite", if you like.
I've often thought that animation would be a useful medium for certain types of stories that might be otherwise difficult or impossible to tell - say, a meeting between the current Doctor and a Doctor from the old show.
[Part of list of Doctor Who episodes here.]