The Doctor Who story that this novel adapts is something of an oddity: a two-episode story set entirely in the TARDIS. The story itself is marred by some very peculiar decision on the acting styles of some of the regulars.
Fortunately, in this novel, we are spared this. This novel largely focusses around Barbara as a mysterious force invades the TARDIS and some of the travellers, particularly Susan, act oddly. Tensions between the characters increase as the Doctor becomes suspicious that Ian and Barbara have caused the problem by endeavouring to return to Earth by meddling with the control console.
The main strength of this book is the insight into the thoughts of the characters, particularly Barbara. While it is possible to watch the TV serial and develop views about what they are thinking, this book takes the guesswork out of this (which is good, given some of the odd performances mentioned above).
The weak point of this book is that it begins (again!) with a recap of the first meeting of the TARDIS crew. If you were reading the novelisations in broadcast order, this would be the third and has the third depiction of this event.
Overall, a good novelisation that surpasses the original.