Doctor Who and the Vortex Crystal has the Fourth Doctor landing on a planet with Sarah Jane Smith and Harry Sullivan, and having to sort out the evil Masters who are oppressing the planet's inhabitants. But the Masters (whose identity is not difficult to work out) have problems of their own, and one crucial forking point in the story requires the reader to correctly work out which pre-Fourth Doctor baddie is likely to be Behind It All. There are some risible bits of running around mazes and exploring networks of corridors; plus the Doctor, as viewpoint character, spends most of the book looking for Sarah and Harry who are therefore are mostly offscene. But it's good fun.
Doctor Who and the Rebel's Gamble takes the Sixth Doctor, Peri and again for some reason Harry Sullivan to three potential turning points of the American Civil War, in order to prevent someone else form turning those points (the lost Confederate orders which helped the Union win at Antietam, the death of Stonewall Jackson in friendly fire, and Gettysburg). Keith actually rises to some pretty good writing here, by Doctor Who 1980s standards, on the horror of war and dealing with grief. He could have made a bit more of Peri being from Baltimore, but otherwise the Doctor's travelling companions get better exposure this time.
They are a bit formulaic, of course, and also, I am too old to bother with making saving throws, and just turned to the best outcome on offer instead. But I did appreciate the internal structure of both books, which rewards the reader/player for taking longer rather than shorter routes to each stage of the story, and even the sidetracks that turn out to be irrelevant are well enough written. An unexpected jewel.