Chris Bohjalian savors the experience of getting behind his characters’ masks, deep into their psyches, such that readers know far more about why the people in his novels do what they do than the characters themselves know.

Typically they live ordinary lives but come to be faced with extraordinary circumstances and pressing moral dilemmas. You feel for them as though they are your friends, watching through the sealed window of the page, wanting to call out to them, to warn them away from taking questionable actions and making risky decisions.

Bohjalian’s newest novel, “The Sleepwalker,” is one of his most skillfully plotted. In truth, it is a mystery. He gives you the heart of the mystery in the first sentence of chapter one: “Everyone in the county presumed that my mother’s body was decaying – becoming porridge – at the bottom of the Gale River.”

The narrator is 21-year-old Lianna Ahlberg, home for the summer in Bartlett, Vermont, between her junior and senior years in college. Her mother, Annalee Ahlberg, a successful architect and a woman of arresting, Nordic good looks, has disappeared. She apparently got up while her Middlebury College professor-husband was away in Iowa at a conference, leaving Lianna and her 12-year-old sister, Paige, asleep in their Victorian home on the edge of the village.

It is no secret in town that Annalee is a sleepwalker. Everyone knows the earlier story of Lianna finding her mother naked, standing on a pillar of the bridge over the Gale River one night, and how she saved her from whatever somnambulistic disaster that awaited.

Each chapter is preceded by what appears to be pages from a diary or journal that reveals the strange world of the sleepwalking mind. The opening entry is certainly gripping. It closes with these lines:

“Or you are teased by a stirring between your legs, then a craving, and you reach for the body beside you. And if no one’s there? You push off the sheets and climb from your bed. You will search out a stranger who will satisfy it. With any luck, you will wake before you find one. But not always.

“It is – you are – vampiric. And while it would be easy to use words like insatiable or unquenchable, they would be imprecise. Because the libertine needs of your sleeping soul will be sated. They will.

“And that’s the problem.”

Lianna was not sheltered as a child, but she isn’t exactly worldly either. Her passion since childhood has been magic, and she has developed something of a talent, doing birthday parties and occasional club performances. It makes for an intriguing subplot in a story that is, at heart, about a disappearance. The reader gets a few glimpses behind the veil of a magician’s sleight-of-hand and a sense of the pivotal act of misdirecting the audience’s attention.

Though the initial search for Annalee is pursued aggressively, nothing turns up but a small square of her nightgown caught on a branch along the river. The uniqueness of the case catches the attention of Gavin Rikert, a youthful detective with the state police who asked to be assigned to the investigation.

Almost from the first encounter, an attraction and fascination is sparked between Gavin and Lianna, despite their 12-year age difference. To complicate matters further, Gavin also suffers from somnambulism and knew the victim as a result of their both seeking help from the same sleep center several years before.

Lianna, Paige and their aloof father, Warren, deal with the inexplicable loss of Annalee through different coping methods. Warren submerses himself in his teaching, then drinks himself into a stupor every night in his reading chair at home.

Lianna smokes a lot of pot and acts upon her attraction to Gavin, telling herself at first that it is, in part, to stay close to the investigation. And middle-schooler Paige spends all her free time swimming to prepare for the start of the competitive skiing season, yet months off.

Both Lianna and Paige have inherited their mother’s nocturnal malady, though apparently have not experienced any episodes recently.

Gavin tells Lianna the first afternoon her mother has gone missing that he knew her mother, quite well, actually. That after their time at the center, they had occasionally gotten together for coffee or lunch, that they were like each other’s private support group. Lianna is surprised by how much he knows about her mom, including intimate things from her childhood.

The little-known, dark world of somnambulism, and its unpredictable impulses, assumes a greater active role as the story progresses, beyond being simply a likely factor in Annalee’s disappearance. This is masterfully advanced throughout by the separate journal-like entries, each entry taking the reader deeper into the nether realm of uncontrollable sleepwalking.

Bohjalian tells an increasingly gripping tale layered with grave moral dilemmas for those who suffer somnambulistic episodes, ratcheting the tension as their behavior takes bizarre turns.

This tension is further intensified by the complex dilemmas that those who love them and wish to protect them face, as they, too, fight feelings of helplessness.

The popular notion that sleepwalking is humorous and harmless is belied by the dire, deadly consequences that entangle the Ahlberg family of sleepy Bartlett, Vermont.

Frank O Smith is a Maine writer whose novel, “Dream Singer,” was a finalist for the PEN/Bellwether Prize, created by best-selling novelist Barbara Kingsolver “in support of a literature of social change.” Smith can be reached via his website: frankosmithstories.com.