The Boston team of Rizzoli and Isles deals with Catholic saints and dramatic murders in ‘I Know a Secret.’

It was perhaps inevitable that Tess Gerritsen would write a novel that prominently features a horror movie. She grew up accompanying her mother to such movies as a kid, and she and her son Josh recently collaborated in the making of “Island Zero,” an indie horror film set on the coast of Maine; Gerritsen, who lives in Maine, wrote the screenplay and her son directed. “I Know a Secret” is another installment in Gerritsen’s best-selling Rizzoli & Isles thriller mystery series. The book opens with Boston detective Jane Rizzoli and medical examiner Maura Isles investigating the murder of a young woman who is found dead in her warehouse loft apartment. This isn’t your typical murder. Twenty-six-year-old Cassandra Coyle, an independent filmmaker, lies on her bed with her eyes cut out, each one placed in an open palm.

When Rizzoli and Isles interview her partners at Crazy Ruby Films, all former New York University classmates, they learn that the four had just finished their second feature horror film, “Mr. Simian.” Coyle wrote the screenplay based on a real-life event from her childhood, the disappearance of a young day-care classmate who never was found.

The plot, one of the partners tells Rizzoli and Isles, is “straight out of Horror 101. Eventually the killer must come after the heroine.”

Not long after, Rizzoli and Isles find themselves standing on a pier over the body of a young man with three arrows in his chest. Like the first murder, this one feels staged.

The story is interspersed with chapters told from the point of view of a woman who is riveted by the mysterious killings. The book opens with her attending a funeral in Newport of a childhood friend who supposedly died in a house fire caused by candles left burning.

Before she heads back to Boston, the woman drives out to the charred remains of the victim’s home. “The wind gusts and dead leaves rattle across my shoes, a brittle sound that brings back another autumn day, twenty years ago, when I was ten years old and crunching across dead leaves in the woods. That day still casts its shadow across my life, and it’s the reason I am standing here today.”

Glancing down at the makeshift memorial of flowers, she notices in horror a single palm leaf amidst the bouquets. She well knows its meaning. It is the symbol of a martyr.

The investigative team is slow to find the thread that links the two killings they’re working on. Then Isles pursues a hunch by contacting her friend, Daniel Brophy, a Catholic priest and her former lover. She shares photos of the two victims.

After viewing the photo of the eyeless corpse, Brophy looks up at Isles in shock. “Saint Lucy,” he says. “That’s exactly who I thought of,” Isles replies. After looking at the second photo, he says, “Sebastian, patron saint of archers and policemen.” Both are martyrs. St. Lucy, patron saint of the blind, had her eyes cut out for refusing to marry a non-Christian. St. Sebastian was shot through with arrows on the orders of Emperor Diocletian, who persecuted Christians (Sebastian survived but was later clubbed to death, again on the emperor’s orders).

Pursuing this line of reasoning, Isles ultimately discovers the death of Sarah Basterash, the fire victim. The pieces begin to fall together. Cassandra Coyle, who had her eyes cut out, was born on Dec. 13, St. Lucy’s Day; Tim McDougal, who was found with arrows in his chest, was born Jan. 20, St. Sebastian’s Day. And Sarah Basterash was born on May 13 – the day dedicated to St. Joan of Arc, who was burned at the stake.

Rizzoli and Isles are stumped. What’s the connection? How many other victims might the killer be stalking? And later, are things really as they appear?

Gerritsen weaves a compelling horror story with a killer loose, methodically coming for his victims.

Mixed in are subplots involving Rizzoli and her highly dysfunctional family, and Isles and her continuing love for Father Brophy, as well as her haunted relationship with her mother and brother, both of whom have dark pasts as hired killers.

Several major surprises await around dark turns in the plot. As does the reveal of how Crazy Ruby Film’s “Mr. Simian,” written by Cassandra Coyle, the first victim, fits squarely into the puzzle.

Gerritsen is faithful in “I Know a Secret” to the dictates of Horror 101 in winding her story to conclusion. And though everything in the mystery is resolved, not everything in the case gets wrapped up tightly.

This is by design. Gerritsen seems to be telling her readers that some horror stories held between jacket covers are unending – just as in life. One can finish the story, but the horror remains.

Frank O Smith is a Maine writer whose novel, “Dream Singer,” was named a Notable Book of the Year in Literary Fiction in 2014 by “Shelf Unbound,” an international review magazine. Smith can be reached via his website: frankosmithstories.com.