Second paragraph of third chapter:
If the forces of darkness became too powerful, the White Guardian was able to use the Key to reset the balance. Otherwise the universe would fall under the control of the Black Guardian and slip into eternal chaos – with Time itself perhaps ceasing to have meaning.
This is really rather gorgeous – clearly rips off the British Museum's excellent podcast and book with similar titles, but a good idea is worth
stealing imitating. Tribe and Goss list 100 important objects in the Whoniverse in chronological order (ie from the early universe to the far future, passing through the 1960s and 2000s en route), mentioned in TV stories from 1963 to 2012; each entry recapitulates the story or stories in which the particular object appears, but then also looks at other stories with similar themes (eg space arks) and even at sources of inspiration for the originating writers. The whole thing is beautifully illustrated. Definitely at the top end of the Doctor Who reference book range. Get it here.