British author Clare Mackintosh’s debut thriller, “I Let You Go,” was an international bestseller in 2015. The book is just out in the United States and comes with the gleeful notice — wait for the twist.

The twist midway through “I Let You Go” is the book’s most notable, and marketable, feature.

The twist here is not the traditional kind of surprise readers encounter in certain cleverly plotted novels. In “I Let You Go,” the author engages in a bit of trickery. While I nearly walked away from the book when confronted with the confounding device, I was too far in, as the title says, to let it go. But I didn’t trust the author going forward and read on cautiously. More land mines ahead?

Twists aside, “I Let You Go,” starts out when a 5-year-old boy is killed in what appears to be a hit-and-run accident on a rainy night in Bristol. Detective Inspector Ray Stevens and his team’s newest detective, Kate, spend months trying to track down the driver, to no avail. They have almost no clues. After five and a half months, their boss tells them to move on. When they briefly reopen the case at the one-year anniversary, a witness comes forward and the detectives encounter a bit of good luck.

DI Stevens is not a likeable guy. He’s into his work and he’s into Kate. His wife and kids are background to him. His son Tom is not doing well in school and Ray misses key meetings with the teacher. Stevens, when he thinks about his son’s behavior at all, makes assumptions that he fails to validate. He’s not the most trustworthy detective, as a result. At work, Kate has the energy and moxie to override Tom’s politically motivated caution. She lights a fire under him that he lets burn out of control. Insert “nudge” here.

We get to know Jenna, too, a young artist who flees Bristol to live quietly and anonymously on the coast in Wales. She walks out on a successful career as a sculptor but quickly finds a new, creative and somewhat lucrative outlet in photography. A self-important businessman is featured in the story, as well. Ian turns out to be a brutal psychopath and Mackintosh spares neither blood nor bone nor family pet to show us the extent of his brutality.

Mackintosh renders a fine portrait of Penfach, where Jenna moves. It’s a tiny coastal village in Wales that, while insular and judgmental, kindly stretches each summer to accommodate an influx of tourists intent on basking at the sea. Jenna spends a happy, healing year in Penfach.

Because “I Let You Go” changes fundamentally halfway through the book, reviewers must exercise care. No one wants to spoil the surprise. Beyond the highly touted twist, additional surprises follow, right up to the end. But rest assured, the big twist happens only once. Knowing this will ease your anxiety as you read, should you be the wary type.

Lee Child, a favorite thriller author who also hails from England, blurbs: “Astonishingly good.”

That’s an enviable endorsement. While DI Stevens is no Jack Reacher and Kate is barely discernable, Jenna is an interesting character. She’s talented, lively and resilient. Ian, on the other hand, displays a cookie cutter kind of malice that, while disturbing and ugly, is sadly known to us throughout the entertainment media. The talented Mackintosh, who spent 12 years in the Thames Valley Police Department working on criminal investigations, is likely to deliver more nuanced thrillers in the future.

— Rae Padilla Francoeur’s memoir, “Free Fall: A Late-in-Life Love Affair,” is available online or in some bookstores. Write her at rae.francoeur@gmail.com Read her blog at http://www.freefallrae.blogspot.com/ or follow her @RaeAF.

“I Let You Go” By Clare Mackintosh. Berkley, New York 2015. 369 pages. $26.