Any thriller’s going to require a certain amount of suspension of disbelief to enjoy the plot, but author Scott Sigler prides himself in minimizing that as much as possible. His latest novel, “Nocturnal,” tells a story that defies classification, mixing police procedural, conspiracy theory and horror.

Although it’s no less ridiculous than other novels in any of those genres, there’s slightly less hand-waving when it comes to the explanations of why things happen the way they do. Does this make a difference? Evidently it does to Sigler, but it’s doubtful most readers will care either way.

Still, the explanations don’t get in the way of an exciting story, which involves several murders in San Francisco, two police officers told not to investigate them and a 13 year-old child who is somehow connected. While the novel starts out as a standard buddy cop story, it takes several turns before reaching the conclusion that most readers won’t anticipate from the first hundred pages.

Sigler writes solid prose and has done his research to make the story as believable as possible, but his greatest skills as a writer are his imagination, ambition and sheer guts.

“Nocturnal” features a huge, citywide scale that holds a candle to some of Stephen King’s bigger novels, such as “It” and “The Tommyknockers.” And like King, Sigler is fearless, constantly breaking unspoken rules — putting children in danger and even brutally killing them if the story calls for it. As a result, the reader learns early on that no character is safe.

Where Sigler could use a bit more work is his characterizations. Many, if not all, of the players in the novel are broadly drawn variations on stock characters we’ve seen hundreds of times before — the too serious police officer with commitment issues along with his goofy, vulgar-tongued sidekick, among others. While archetypes can be useful and even efficient tools, there are story elements that suffer here as a result.

The worst victim is the love story between the main character, Bryan Clauser, and his ex-girlfriend Robin Hudson, which never comes off as more than a distraction from the main plot. It’s surprising how often characters who live every second of their lives on the run from brutal killers find time to spark up or rekindle romantic relationships.

However, the characters aren’t the focus of “Nocturnal,” and neither is the believable science. Sigler is ambitious in even trying to explain the fantastical elements of the story, but he knows not to let it get in the way. Instead, like Dean Koontz on his better days, Sigler uses the science decoratively to provide explanations here and there during the gruesome and horrific story rather than as the backbone that it rests upon.

On one hand, it’s nice that the science doesn’t get in the way of the story. On the other, it’s a tad disappointing that this science doesn’t add up to a whole lot, outside of a few expository scenes. Still, there are plenty of great non-fiction books out there for those with an interest in learning about the science on display here, some of which are included in a list at the end of the book. For those who want a thrilling and fantastical tale of murder and monsters in San Francisco, however, “Nocturnal” should do the trick.