What could be scarier than learning the man you've shared a life with for 30-odd years is a serial killer?

It’s Stephen King territory, and the horror novelist’s first screenplay adapted from one of his works in 25 years tackles that scenario head on.

Only “A Good Marriage,” out today in select theaters and Video on Demand, isn’t interested in the usual King theatrics. It’s a study of marriage, the tiny gestures and role playing that make up the institution. It’s also a mystery since the wife’s reaction to her husband’s crimes is far from what we expect.

 

Joan Allen is Darcy, mother of two grown children and wife to a man everyone sees as warm, loving and loyal. Their marriage appears perfect, or as perfect as any long-term bond could realistically be.

Bob (Anthony Lapaglia) has an annoying habit of leaving notes around the house telling his bride not to eat too much or otherwise ruin her figure.

He has a secret beyond his minor control issues. He’s the man behind a series of gruesome slayings plastered all over the local news. One day Darcy discovers the driver’s license of one of the killer’s victims stashed in Bob’s belongings. She digs a little more and finds indisputable proof Bob is leading a dark double life.

Now what? Does she call the police? Confront Bob? Or try something else that someone like King might explore?

The answer lifts the film from its clumsy setup into an intriguing arena. And then … nothing. The film’s middle section sags heavily, squandering the story’s potential and making us wait for the next shoe to drop.

By then it’s too late, even if the ending packs a matriarchal tenderness Allen delivers with restraint. King’s screenplay hums in that final act, his cheery sense of wordplay coming to life as if woken from a deep slumber.

Adapted from King’s short story “A Good Marriage” from “Full Dark, No Stars,” the film highlights how a long-married couple’s rhythms keep their connection alive. When Darcy finds a copy of “Bound,” a hardcore sex publication, in Bob’s belongings she greets the news with a shrug. It’s a beautifully forgiving moment, showing how a loving partner will forgive her love’s unflattering side while acknowledging all the good he brings to their lives.

That tenderness doesn’t last long, and soon we’re left to wonder just what is real and what is pouring out of Darcy’s fractured psyche. It’s a visual dance done few favors by director Peter Askin (“Trumbo”), who can’t create a sense of dread while teasing the storytelling device to its practical conclusion.

“A Good Marriage” ultimately feels small, like a meaty King riff stretched beyond its capacity.

DID YOU KNOW: Stephen King applauds a select number of films adapted from his works, including “Carrie,” “The Shawshank Redemption” and “The Green Mile.” He calls “Maximum Overdrive,” the only film he both wrote and directed, a “dog.”