SK Tremayne is the nom de plume of a best-selling author and award-winning travel writer who, for the time being at least, wishes to remain anonymous.

This is a stratagem that is sometimes employed by writers who wish to radically change gear or genre, allowing them to break new ground while avoiding any preconceptions based on previous work.

Tremayne's tale opens in London with a couple in crisis. Sarah and Angus Moorcroft have lost Lydia, one of their young twins, in a tragic accident and the stress is taking its toll on their marriage. Then Angus is left Eilean Torran, a tiny Hebridean island, by his late grandmother, and he and Sarah decide to take their surviving twin, Kirstie, to live there. Angus knows the island well from his childhood holidays and Sarah is caught up in his enthusiasm.

This introductory section of the novel works well as the couple are seduced by the urbanite's fantasy of living a cleaner and simpler life away from the noise, dirt and crime of the big city. When they arrive, the reality is sobering as they realise the island's old cottage is "a brutal nightmare", so overrun by rats that food has to be stored in baskets hanging from the ceiling.

Just before they leave London, seven-year-old Kirstie begins to act strangely. "Why do you keep calling me Kirstie, Mummy?" she asks. "Kirstie is dead." At first Sarah humours her daughter, believing that she is confused and this is part of her grieving process. After all, the twins were very close and startlingly identical, with blue eyes and pale blonde hair.

Once on the island Kirstie becomes more and more insistent that she is Lydia. At the local school she frightens the other children with her strange conduct and they retaliate by chanting "Bogan" at her, which is Gaelic for ghost.

Kirstie/Lydia's confusion over her identity and her erratic behaviour are both strange and moving. As she comes to terms with losing her closest companion, she also has to cope with her parents' marriage dissolving before her eyes. She is anxious and bewildered by her own actions, making her by far the most sympathetic character in the novel.

As her relationship with Angus disintegrates, and Kirstie/Lydia becomes more emotionally distant, Sarah feels "lost in the hall of mirrored souls". Things come to a climax during a ferocious storm that leaves Sarah and her daughter stranded on the island as the wind and rain batters the cottage, ripping holes in the roof.

Tremayne uses some powerful metaphors to enrich the atmosphere of the story. The island's name, Eilean Torran (Thunder Island), introduces a sense of foreboding and its isolation echoes the family's isolation, encapsulated as they are in their own world of grief. The dilapidated condition of the cottage reflects the state of Sarah and Angus's marriage, something that needs love and care to bring it back to life; and the menacing graffiti figures painted on the interior walls of the cottage by recent squatters are like the ghosts that haunt the family's relationships with each other.

By introducing elements such as supernatural occurrences, wild storms, dark secrets and Sarah's increasing dread, Tremayne builds the unsettling mood of the piece. The astonishing beauty of the landscape forms a stark comparison to the ugly thoughts forming in Sarah's mind as she begins to suspect that her husband has been lying to her for some time.

At times the unfolding of the novel's plot is plodding and lacking a sense of urgency. The language is occasionally clunky as, for example, when a photo shows Kirstie and Lydia looking "desolatingly happy". Thankfully the locals are not depicted as couthy or twee but as ordinary people living in an extraordinarily magnificent but challenging part of the world.

After the complex and dramatic build up, the resolution to the story is a little disappointing. It concerns a piece of information that puts a new slant on what has happened to the Moorcroft family and, while it makes sense, it seems incongruous that it hasn't been revealed before.

This is a pity as the advantages and hardships of island life are well depicted and the Gothic mood is suitably creepy. While Sarah, who narrates large parts of the story, is an appealing character, it is the fate of the troubled Kirstie/Lydia that intrigues.