The first story in the Key to Time season, this book adapts a number of important events: the meeting between the White Guardian and the Doctor, which commences the sequence, and the arrival of new companion, Romanadvoratrelundar (or Romana for short, even though she's rather be called Fred...).
Adapted by Ian Marter from a great script by Robert Holmes, the attention to detail is good and the reader is treated to something other than a bare-bones adaptation, as is so often the case with these novelisations.
While perhaps the simplest story in the Key to Time sequence, I don't see this as a fault. Indeed, it allows the reader to relax into the more complex ideas introduced at the start of the book and get to know Romana, without bombarding us with some of the more convoluted story lines that later became normal for this season.
(Incidentally, the second story in this sequence, 'The Pirate Planet', was never turned into a novel.)