"There was someone in the house." The first sentence of Tracy Sierra's debut novel is as bold as a headline and as attention-grabbing as a warning sign. It does what it is designed to do: piques our interest, stokes our fears and forces us to keep reading. From the get-go, "Nightwatching" is a taut psychological thriller in which a mother finds herself pushed to her limits to stay alive and protect her children from an uninvited guest.

Sierra's nameless heroine realizes an intruder is prowling around her creaky New England colonial one snowy, stormy night. She hopes it's her husband but dismisses that as impossible. She then thinks it could be her 8-year-old daughter, sleepwalking, but eliminates that as unlikely. At the top of the stairs, a tall stranger appears, wearing white plastic gloves and carrying an unidentifiable weapon. But is he really a stranger? "His presence had the distantly familiar rancidness of something wrong and rotten she'd tasted before but couldn't quite place."

With no phone or gun at hand, the woman wakes her daughter and 5-year-old son and stealthily takes them downstairs. All three take refuge in "the hidden place," a small, cramped secret room of "walled-in emptiness" behind a brick oven. The woman tries to comfort her terrified children; she also attempts to quell her own panic with a mantra: "A den, not a tomb. A den, not a tomb."

She watches her home invader through a vent and listens as his mild-mannered entreaties for money sour into sinister threats to "bring out the bad guy." Eventually, after weighing her limited options, she breaks cover, heading out alone and barefoot in thick snow to search for help. Little does she know that her troubles are only just beginning.

Sierra, an attorney who was born and raised in Colorado, writes with aplomb. She keeps her reader on board by placing her protagonist in one predicament after another, creating an atmosphere of claustrophobic dread and ratcheting up the suspense levels. At certain moments she intercuts her main narrative with flashbacks that reveal the hardships her character has endured, from her father-in-law's verbal "eviscerations" to her mother's death at the hands of a drunken driver.

Sierra also lures us on with a series of unexplained mysteries. What has happened to the woman's photographer husband? Who is her intruder? And is that man a clear and present danger or have trauma, paranoia and sleep paralysis conjured up a phantom bogeyman?

"Nightwatching"

This last puzzle is one of the book's key strengths, particularly when male police officers enter the picture to pick holes in the woman's story and question her sanity. From this point on, "Nightwatching" grows in stature and becomes not just a nerve-shredding page-turner but also an ingenious guessing game and an absorbing account of a woman's struggle to make her voice heard.

Malcolm Forbes has written for the Times Literary Supplement, the Economist and the Wall Street Journal. He lives in Edinburgh, Scotland.

Nightwatching

By: Tracy Sierra.

Publisher: Pamela Dorman Books, 355 pages, $29.