The Fish, the new novel by Commonwealth Writers' Prize-winner Lloyd Jones, takes this simple but shocking idea and teases it out through the iterations and adjustments of family life in his native New Zealand. It is a strange but ultimately beautiful story, marked by tragedy and human warmth.

The unspoken feeling that runs close to the surface throughout the novel is that no one wants to have given birth to a fish. This is not a life that anyone would choose. The father is out of the question, and the narrator's 19-year-old sister is holed up in a caravan with the newborn. "Sticking a teenage girl in a caravan to serve out her pregnancy may seem callous, cruel even. However, I don't remember thinking that at the time. To a child there is a simple logic to the world," the narrator - unnamed through the novel - remembers.

When the Fish is born, his grandparents and the narrator, a young and newly minted uncle, try to be welcoming, warm. The Fish is named after his grandfather, who struggles to relate to his grandchild. The family clashes with the Fish's mother - their daughter and sister.

"The Fish's presence is defined by a set of contradictions. On the one hand, we defend Colin Montgomery's fishiness. But then we wish Colin was not so wholeheartedly a fish - but a bit more like," the narrator says. There is no doubt in these pages that the Fish is a fish: Jones repeatedly draws the reader to the gills, the gulping, the bulging eyes. But this is really a novel about what happens when someone not like us joins the family. The idea of a fish growing up as a boy alone would hardly sustain a novel, but this goes well beyond that.

Jones' Fish reveals how people react or grow to care for someone who is not like them. It is the ultimate test of character, really. Cliches do have a kernel of truth to them, and the expected one finds its way early on into this book: the old idea of what happens to a fish out of water.

The narrator, a young lad, grows, too, with the book. His "glamorous older sister", Carla, leaves early in the story for a new life in Sydney, so as a nine-year-old he writes to her, her "eyes and ears" at home. Carla's letters to New Zealand from Sydney highlight the cultural isolation of the narrator's world. Carla writes excitedly of chocolate milk and blueberry muffins, delicacies of the Australian mainland. The narrator begins to struggle writing back, not wanting to worry his sister with the difficulties of growing up with a Fish. He is really struggling to comprehend his own family's life and circumstances.

The narration of a child, even a young teenager, can be a difficult restriction to a novel. They are inherently unreliable, unable to grasp the complexities, and harsh realities, of adult life. However, Jones' narrator is recalling these events as a much older man, and slips between tenses to add more to the narrative than a child would know. This way the novel captures both the child swimming through the situation in an effort to comprehend and the reflections of an adult. The effect is like looking at a world blown in a glass paper weight; it always feels real, though it shifts depending where you look from. Carla's life in Sydney is both that of a "glamorous older sister" and a "professional girlfriend" with obligations to an agency.

A critic in New Zealand has already pointed out this novel does have anachronisms. The narrator is paid by an elderly neighbour in dollars and cents before the country went decimal, for example. I usually find anachronisms terribly annoying, but I was probably far enough removed from this history to spot them. It is harder to convince locals to suspend disbelief than blow-ins from across the ditch, after all. I'm also not convinced they are a problem here, given the story is rooted in recollection and reconsideration, and no narrator, real or imagined, has perfect recall. There are no glaring historical impossibilities to trip up the reader, I think, and besides, in many ways, this is a novel that exists out of time.

The Fish speaks only twice. This is a novel about the Fish, without his own side of the story. He can certainly make decisions for himself and works hard at his grandfather's scrap yard. In being seen only through other people's eyes, the Fish's presence reveals more about those who watch him, raise him and, in the end, love him. Some cope better than others when someone so unlike them is born into the family. There might be some temptation to read this as an allegory: compare the reaction to the Fish to the reaction a disabled child, or one who is otherwise physically different to their peers, would receive.

Jones' characters are ultimately decent people. They try to do the right thing; they're caring often in spite of themselves and their prejudices, across lives that know tragedy and difficulty.

The prose is elegant and honest. Each sentence is set down with purpose and clarity. The later fragments which border on poetry hint at the ultimate incompleteness of memory and do well to finish off this story.

In the end, it's hard to know what to make of a fish who grows up as a boy in New Zealand more than five decades ago. But I suspect that is the point: Jones certainly makes you wonder, as you reflect on your own life, what would it be like if...?