Fiona McFarlane’s “The Night Guest” is one of those psychological suspense novels in which readers know something is up but aren’t always clear about what exactly it is. There may or may not be a critter prowling around the main character’s isolated beach house at night; her peculiar new caregiver may or may not be on the level; and a murder may or may not have taken place by the end of this tale. Amid all these ambiguities, the one certitude about “The Night Guest” is that it’s a superb first novel that investigates the terrors — both extraordinary and mundane — of old age.

Ruth is in her mid-70s and living on a lonely stretch of seacoast in Australia where even surfers are scarce. It was her husband’s idea to retire in this secluded spot, but he keeled over shortly after they committed to the move. Lately, her mind has been wandering back to her childhood in Fiji, where she grew up as the daughter of missionaries. Uninterested in food, Ruth nibbles on pumpkin seeds for sustenance and dotes on her cats for company. Perhaps she’s become a bit too susceptible to their feline charms because she’s started to suspect that a bigger cat — a tiger — is roaming around her house in the dead of night. She smells its rank odor and hears its breathing, “thick with saliva.”

One night she screws up her courage, ignores her bad back and ventures out of her bedroom. Dull as her life otherwise is, Ruth is excited by the new sensation that something of “extravagant consequence” may be about to happen to her: “She felt something coming to meet her — something large, and not a real thing, of course, she wasn’t that far gone — but a shape, or anyway a temperature. It produced a funny bubble in her chest.”

Turns out there’s barely a trace of a tiger, tiger burning bright on the premises (that time), but a strange shape does indeed approach Ruth as morning breaks: A large woman in a thin gray coat appears at her door and announces herself as “a government carer” named Frida Young who’s there to look after Ruth every day for an hour as part of “a state programme.” Apart from its many literary virtues, “The Night Guest” stokes suspicions in a way that will gladden the hearts of small-government proponents everywhere.

McFarlane’s un-showily poetic style enhances the hallucinatory atmosphere. Take this enchanting description of Frida’s housekeeping prowess: “The house took to Frida; it opened up. Ruth sat in her chair and watched it happen. She saw the bookcases breathe easier as Frida dusted and rearranged them; she saw the study expel its years’ worth of . . . paperwork.”

"The Night Guest" by Fiona McFarlane. (FSG)

As weeks pass, Ruth becomes increasingly dependent on Frida, who’s grown moody. Unnoticed at first by Ruth, Frida has also moved into a spare bedroom. When did she start spending the night? Did Ruth forget that she agreed to this plan? And if the tiger that Ruth still hears stalking around the house at night isn’t real, why is Frida mopping up clumps of wet sand every morning?

Being elderly and confused makes Ruth an easy target for predators (both feline and human), but McFarlane is wise about the secondary consequences of aging that can leave pensioners vulnerable. Death, illness and distance have robbed Ruth of the protection of family and friends; in fact, her two adult sons, who live far away, are so relieved at the thought of a caregiver who’s magically appeared (like Mary Poppins!) to relieve them of worry about their mother’s everyday needs that they don’t even question Frida’s credentials.

The only place where “The Night Guest” stumbles is in its conclusion: There, sinister ambiguity tips over into frustrating murkiness. Instead of uttering a breathless “Aha!” readers will most likely find themselves muttering, “Huh?” Should “The Night Guest” be made into a movie — and this tale has “Judi Dench vehicle” written all over it — red herrings will need to be tossed out and a more fitting resolution found. Aside from the final misstep, this is an unsettling funhouse ride deep into a territory that even the brave don’t enter willingly.

Corrigan, who is the book critic for the NPR program “Fresh Air,” teaches literature at Geogetown University.