'Timewyrm: Apocalypse': Now With Unicorns!
That might be a slightly unfair summation of 'Cat's Cradle: Witch Mark', but only slightly. Once again, we have a series of extremely familiar Doctor Who tropes linked together into a novel--an alien world linked to Earth via some sort of time-space corridor, a sinister conspiracy of rustic yokels on the Earth end of the corridor aiding the aliens, strange happenings that draw in the Doctor, various squabbling factions of humans/aliens who need to unite against a greater threat, and the whole thing proves to be an enormous alien experiment and everything we've been told turns out to be a myth handed down that corrupts the truth of the situation.
Once again, we have a Doctor and an Ace that aren't quite on-model. Hunt writes the Doctor as though he's seen Season Twenty-Four and had everything after summed up for him, and Ace...okay, I'll grant you that it's an interesting idea to do something with Ace's experiences on the planet of the Cheetah People, and to play with the idea that she's no longer fully human. But that's the sort of thing that an editor should co-ordinate, so that it doesn't suddenly pop up seven books into the series for about a page or so, never to be discussed again.
And once again, we have only a vague and tangential connection to the story arc. Admittedly, this is shared by the other two novels in the story arc; really, the only reason we know this is an arc at all is that we're told by the title. (It'd be more accurate to suggest that this is the beginning of an arc that continues through the next few novels, as the Doctor's actions at the end have some lingering consequences, but this is also the end of catchy "pre-title" titles for the story arcs, so from here on out we're going to have to go on fan consensus as to when things start and stop.)
As with 'Apocalypse', it's not actually bad, per se. Filled with random moments of fanwank, kind of dull in a manner peculiar to mediocre fantasy novels (this isn't the best place to get into a full discussion of fantasy tropes, but as this does mark the beginning of the pseudo-fantasy sub-genre, it's worth mentioning that they all seem to involve tramping through lots of wilderness and getting into random encounters a la Tolkien and every third D&D campaign, and as much fun as it can be to see the Doctor subvert the tropes of other genres, he never quite seems to do enough to subvert fantasy stories. Just once, I'd like to see him skip the whole thing by materializing the TARDIS on top of whatever this week's equivalent of Mount Doom is.) And a lot of the supporting cast seems to be just hanging around, desperately hoping that the novel will be popular enough for them to be in a sequel. But it's harmless enough, and doesn't actually have any real clunkers in terms of plotting or dialogue.
This one really feels like it's just marking time, waiting for something better to come along. Fortunately, something does.