It's one hundred years post the end of the tv story planet of evil. For the people of the planet that the spaceship in that story came from, at any rate. Left by the fourth doctor to find a new source of energy, a hint he gave them has had consequences.
Drawn to the world by visions of disaster, the fifth doctor must deal with this. And the result of something his former self did.
This is the best kind of sequel, because it takes the story of the original and spins it off in a new direction. The tardis crew are very well characterised, and the story is compelling. A very entertaining read and a rather underrated book