It might rely a BIT too much on continuity- knowledge of "Image of the Fendahl" and the Future War are a must if you're going to understand what's going on here-, but the plot is INCREDIBLE; the Doctor faces a nightmare from the dawn of history that will be unleashed by his own people, while simultaneously trying to figure out who created a species that shouldn't even exist...
Hell, even the TARDIS gets some interesting characterisation here (In the sense of its opnion of the Doctor is explored and demonstrated in depth as the Doctor confronts other TARDISes)!
And as for the monster...
THIS is why you read novels, really; the scale of the creature here is so incredible it could only be aptly expressed in the imagination.