In the story, lycanthropes are real and less limited than simple old-school werewolves. They can transform into almost anything, are invisible to most humans when transformed, and manipulate probabilities (what today we'd more likely label quantum uncertainties) at will, enabling them to walk through walls and arrange nasty 'accidents.' However, for the most tenuous of hand-waving reasons, dogs and silver still pose a mortal threat to them.
Will Barbee is the protagonist, a decent newspaper reporter and alcoholic. The story starts as an expedition returns from an H. R. Haggard story--or rather, from an archeological/anthropological expedition in the deserts of Asia. The leader of the expedition, once a mentor of Barbee's but since estranged, begins to make a dramatic announcement, but dramatically falls dead in the middle of it. His younger assistants, contemporaries and friends of Barbee's, cut short the press conference with a show of "nothing to see here," and set about securing a green wooden MacGuffin.
Using his instincts, Barbee quickly determines that a new reporter he met at the conference, a woman wearing white fur named April Bell, is responsible for the doctor's death--she was carrying a kitten (the doctor was allergic to cats), and Barbee finds the kitten strangled and stabbed with a pin. Despite this rather disturbing scene, he becomes besotted with April Bell and starts trying to learn more about her.
Next, he begins having dreams where she calls to him, and he turns into various creatures, follows her, and helps her kill the other people involved in the expedition. It turns out that in ancient times there was a war between homo lyncanthropus and homo sapiens, which 'normal humans' more or less won. However, the lycanthropes are regaining strength, and April Bell enlists Barbee to help make sure that the anti-lycanthrope weapon the expedition brought back from Asia in the green wooden box is destroyed.
Barbee spends most of his time being psychologically torn in many directions. He's in love-or-lust with April Bell, despite the fact that everything he can find out about her paints a very unpleasant picture of a woman who is either a witch or psychotically disturbed. During his dreams of being a werewolf (or were-sabre-tooth-tiger, or were-snake, etc.) he is torn between arguing to save his friends and killing them. He checks himself into a mental institution and is torn between the fact that his dreams seem real (and the consequences are absolutely real), but everything he knows to be true about the natural world argues that lycanthropy is impossible.
Williamson's telling of Barbee's inner conflict makes this book unsatisfying and frustrating. It's obvious from the narration that Barbee's dreams are real--there are no conditionals about the language used (Barbee 'does' this and that, instead of 'feeling' like things are happening, or feeling like things 'might have' or 'could have' happened). The reader obviously is meant to understand that the fantastic explanation is the correct one, so when the psychologist explains how all this would look under a non-supernatural Freudian analysis, it is plain to us that it is so much obscuring fluff. However, it takes until the final pages of the book for Barbee to come to terms with the reality of lycanthropy and witchcraft. He spends almost the entire narrative waffling between the different poles of his inner conflict, and having general histrionics about the events he's involved with. That's a valid narrative choice--in the real world, I imagine most people would react the same way. However, for the genre reader to whom things like lycanthropy are more-or-less routine, I kept wishing that Barbee would get with the program, realize what's going on and how he's being manipulated, and seize some control of the situation. It is frustrating to read about him waffling back and forth, and disregarding really disturbing evidence, while allowing himself to be used by the very unpleasant (but apparently gorgeous) April Bell. And let's not even get into the fact that in the world-building background of the story, the Inquisition and witch-hunts were perfectly legitimate endeavors to protect humanity from a racial threat, and thus that materialist skeptics/humanists are enabling this racial threat to re-emerge by not believing in the supernatural. That's a position that I think any author in the last 30-40 years would be very hesitant to include.
So this is another classic genre piece that falls into the 'I'm glad I have finished reading it' category. As with so many seminal works, the story leans heavily on the novelty of the concept, and to readers for whom the concept is not only routine but almost cliched, the story becomes a bit tedious. This makes it harder to over-look the casual misogyny and endorsement of historical mass murder embedded in the structure of the tale. I think that the whole thing could have been more effective if the uncertainty of 'is this a dream or is this real' had been strengthened and sustained longer, but that may not have been possible when playing with some of these tropes for the first time. I like some of the world-building elements: the fact that the lycanthropes manipulate probability for their powers, their ability to turn into any number of animals, and the meshing of the world-building with the tenets of Freudian psychology was definitely novel. But overall, this story ends up being less than the sum of its parts.