As Will Barbee waits, he's approached by a beautiful stranger. She's April Bell, the new reporter for his employer's rival, the Clarendon Call. Strange events attend their meeting. The guide dog for Mondrick's blind wife attacks April for no apparent reason. Then, when Mondrick and his assistants alight on the tarmac, they seem not elated but terrified. And Mondrick makes the bizarre announcement that he has discovered why evil exists: A secret enemy resides among humanity, interbreeding with Homo sapiens as it awaits the coming of its "Black Messiah," a "Child of Night" who will spark a rebellion by the prehuman race. Before Mondrick can explain his wild assertions, he drops dead. And Barbee suspects April Bell is responsible for the death.

For both professional and personal reasons, Barbee tracks the woman down. In response to his questions, she claims that she's a witch and used her power to kill Mondrick and prevent him from proving the existence of her kind. Her strange story only increases Barbee's fascination, and, when Mondrick's widow warns him that April is more dangerous than he can imagine, he refuses to keep away from her.

Barbee's nights become haunted by nightmares. He dreams that he slips out of his body and runs through the night as a wolf, as a saber-toothed tiger and as other powerful beasts, all with preternatural powers. And always April Bell runs with him, as a white wolf, or as a naked woman astride his back. She tells him that they're members of a separate species, Homo lycanthropus—a species with powers that let them change their shapes and influence probability. And when Barbee and April roam the night, they use their powers to kill someone connected to Mondrick's expedition, which, April asserts, has recovered an artifact capable of destroying Homo lycanthropus.

Barbee wants to believe his nightmares are nothing more. But the dream-murders bear a remarkable resemblance to mysterious nighttime killings in the real world. He fears that April's and the Mondricks' crazy tales are sober fact. And he finds himself fighting an overpowering compulsion to shapeshift every night, and to butcher those who would destroy Homo lycanthropus, even though they're his friends. Will he side with his friends and with humanity, or will he serve the Child of Night and the horrifically fascinating woman who urges him to the slaughter?

Is this sci-fi, fantasy or horror? Yes.

More than any other SF writer, Jack Williamson (1908-2006) was a pioneer. This vastly influential author, whose nearly 80-year career spanned essentially the entirety of the modern SF era, arrived in New Mexico in a horse-drawn covered wagon. He was the second writer ever to be named an SFWA Grand Master (after Robert A. Heinlein), and he received the World Fantasy Award for Life Achievement. He coined the terms "genetic engineering" and "terraforming," which have spread beyond SF to world culture. His classic SF novel The Humanoids is, after Isaac Asimov's "positronic robots" series, the most influential American work about robots. Best known for his SF, Williamson also wrote fantasy and horror. His werewolf novel Darker Than You Think straddles the three genres, exerting a profound influence on them all.

First published in 1940 in John W. Campbell's pulp fantasy magazine Unknown, Darker Than You Think was expanded into a novel published by Fantasy Press in 1948. This genesis leaves its marks. The plot seesaws a little too often between the protagonist's uncanny experiences and his denial of same. Barbee's fling with psychotherapy drags on rather long. Characters withhold information it would make more sense to reveal. The author tries to evoke horror by telling, not showing, with the result that the protagonist spends an awful lot of time "shivering" and "shaking." And Williamson relies too heavily on pulpy "said bookisms"; his characters are forever muttering, protesting, whispering, purring, yelping, etc., and Will Barbee gasps so frequently that readers can't help wondering how he manages to speak at all. Additionally, the work's influence guarantees that some of its inventive elements, so exciting when it first appeared, have become overfamiliar through the borrowings of later works.

However, Darker Than You Think retains its power into the 21st century for a number of reasons, including its transcendence of two-dimensional pulp characterization. Will Barbee is no stock pulp-SF hero, whipping his problems and winning the girl with his superhuman sword-fighting talent, his superscientific radium gun or a superpowered invention of his own supersmart devising. Instead, Barbee has a more realistic mix of strengths and weaknesses. He's a reporter with a "nose for news," but he sometimes fails to ask the right questions. He's confident enough to pursue an intimidatingly beautiful and sinister woman, but he falters when it's time to advance the relationship beyond drinks. Denied his calling, he forged a new career but made no attempt to stay in his original field. Like many a pulp hero, he has a weakness for alcohol—but how many other pulp-era protagonists are strong enough to seek therapy when their situation gets beyond them?

While antagonist/love interest April Bell is not as complex as Barbee, she is, as her dual status suggests, a well-drawn character. The author sketches such a beautiful and diabolical portrait that readers share Barbee's fascination. Williamson pulls off a characterization that is common in noir fiction but rarely achieved in pulp SF, fantasy, or horror: the femme fatale. When the femme fatale is properly developed, as in noir classics like Double Indemnity and The Postman Always Rings Twice, the protagonist knows or suspects that he should stay away from her, yet he cannot. He returns and does what she wants—and is, more often than not, destroyed by her.

This leads to Williamson's finest achievement in Darker Than You Think: the ending. It's an uncommonly dark one for the pulp era—so dark that it might have been penned in 2007. And, while it's entirely pellucid, it's also disturbingly ambiguous. Whether the protagonist is destroyed or redeemed, and whether the ending is unhappy or happy, are left for the reader to decide.