'The Doctor and Jamie arrive in Prague and discover that time is out of synch, as different time periods begin to appear, moving backwards in time. The Doctor soon realizes that even his notebook, a book about every place in history, is slowly dissolving in his hands. Time is being erased. He realizes that it is a Doredsor, an creature as old as the universe and that feeds on time. Prague shifts to the point before the city even existed, about 50,000 BCE. The Doctor and Jamie find the Doredsor and discover it is dying. Jamie pleads with the Doctor to help it. In the end, the Doctor allows the Doredsor to feed on the TARDIS just long enough and then it falls into the sun and dies.'
[The Doctor is the Second Doctor.]
Source: drwhoguide.com (http://www.drwhoguide.com/whotrip23.htm)