Readers of Rose Tremain's 11th novel who find themselves inspired to rush off to the French countryside she lovingly conjures will hardly be able to claim they haven't heard the warnings of "buyer, beware" that nestle between the vivid descriptions of brooding hilltops and babbling streams, particularly if they feel inclined to take their chequebooks with them and acquire a prime piece of real estate. In her first novel since the Orange prize-winning The Road Home, which told the story of an eastern European's journey through a bewildering and inhospitable contemporary Britain, she turns to the mountains and villages of the Cévennes to bring us a different vision of cultural collision and the experience of the outsider.

The most significant outsider is Anthony Verey, a once-renowned antiques dealer from Chelsea who finds himself in the shadow of "a universal letting-go"  of fame, money, vigour and desire. Sitting in his forbiddingly elegant shop, kept permanently chilly to lengthen the life-span of "the beloveds", the collective name he bestows on the acquisitions he fears he will miss most in death, he is a man in need of escape, which obligingly arrives in the shape of his ever-dependable sister Veronica. A garden designer enjoying a late-flourishing love affair with Kitty, a mediocre watercolourist in southern France, Veronica has admitted few passions into her life aside from Susan, the horse which mitigated the miseries of her childhood, but Anthony is one of them; and when he decides that what will transform his old age is a splendid house in the Cévennes, Veronica throws herself into making his sketchy dreams a reality.

Such a bond of sympathy and co-operation does not exist, however, between the narrative's mirror brother and sister. While Anthony and Veronica clip unruly olive trees into a semblance of good behaviour and share glasses of chilled white wine on the terrace, Aramon and Audrun Lunel glower at one another menacingly, he from the decaying family house left solely to him by their father, she from the hastily thrown-together bungalow he has allowed her to build on a scrap of land. Now, dizzied by reports of sky-high property prices fuelled by affluent foreigners, Aramon is determined to sell the Mas Lunel to the highest bidder  even if it means demolishing his sister's humble house. Told from childhood that she was "no good on the land", Audrun has only the chestnut and oak wood that was her part of the inheritance, but of all the novel's principals, it is she who has the most respect for her environment.

Tremain sets her story up for disaster, and disaster dutifully arrives on the scene. She builds for each of her characters a delicate backstory of painful endurance and disappointment, from the graphic violence and incest visited on Audrun by her father and brother to Anthony's near-romantic love for his careless and selfish mother. Then, engineering them into an impossibly volatile situation  kickstarted by Anthony's immediate attraction to the crumbling Mas Lunel, and Audrun's determination that it should not be sold  she leaves them to reap the consequences of their wonky desires and their impetuous actions.

Trespass works best through its silences; we feel horribly, for example, for Kitty, who is never allowed to give free rein to her jealousy of Anthony's relationship with Veronica, and who must cope, to boot, with being a rotten painter. Similarly, the minor characters at the edges of the novel  the mayor who lectures Kitty and Veronica on their profligate use of water in the garden, or the Parisian schoolgirl whose alienation from her new rural home tops and tails the story  provide an articulate commentary on our relationship to our surroundings. "They both knew that it was borrowed," writes Tremain of Kitty and Veronica's fragile sense of belonging. "Because if you left your own country, if you left it late, and made your home in someone else's country, there was always a feeling that you were breaking an invisible law, always the irrational fear that, one day, some 'rightful owner' would arrive to take it all away, and you would be driven out . . ."

Insofar as Trespass sets itself to explore the nature of outsiderness and its relationship to our more nebulous yearnings, it is a successful novel, well made and written with a light touch. But it can also appear strangely underpowered, plotted too tightly to its course and prevented from straying into genuinely interesting territory. Tremain has written more freely in the past and, although this may well prove among her most commercially viable novels, it is not one of her most daring.

When he travelled with a donkey in the Cévennes mountains of south-central France in the 1870s, Robert Louis Stevenson took a revolver with him, in case the locals were unfriendly. In her new novel, Rose Tremain vividly evokes the same verdant and recalcitrant region. At the heart of her story are a French brother and sister, Aramon and Audrun, born after the second world war: the progeny of a generation traumatised by loss and accusations of collaboration with German occupiers. By the time the siblings reach late middle age in the early 21st century, when the novel is set, "thousands of Cévenol people had seemed to forget their role as caretakers of the land. Diseases came to the trees. The vine terraces crumbled. The rivers silted up. And nobody seemed to notice or care."

Aramon is a decrepit alcoholic who hopes he can make 475,000 by selling his majestic, if subsiding, stone house, the Mas Lunel. But potential buyers are deterred by the fact that his sister owns the surrounding woodland and has built a squalid modern bungalow on the borderline that allegedly separates her territory from her brother's. Until this dispute is settled, the local estate agents cannot make their killing from a lucrative foreign sale. Theirs is a race against time as the worldwide recession deepens, and the local mayor declares that "displacement of local people by foreigners must end".

From London comes Anthony Verey, a famous antiques dealer in his 60s, suddenly failing to make money from his well-tutored eye for "dead people's" furniture and artefacts. His sister, Veronica, a struggling writer, and her lover, Kitty, an amateur watercolourist, have already emigrated to the Cévenol, and Anthony's arrival brings disruption to the idyll they have found in France.

Aware that the sibling bond between Anthony and Veronica long predates her own involvement in her lover's life, Kitty asks herself: "Doesn't every love need to create for itself its own protected space? And if so, why don't lovers understand better the damage trespass can do?" Tremain traces the role Kitty's jealous anticipation of damage plays in endangering all she most cares about.

Another form of trespass lies buried in the bitter history between Aramon and Audrun. After their adored mother's death, Aramon was encouraged by their father to join in abusing Audrun, the household's one remaining female member. Trespass, in the sense of sin or wrong-doing, has poisoned the atmosphere in the Mas Lunel ever since. Starving and neglected hunting dogs, penned close to the ruined house, are symptomatic of the desperate squalor that has overcome Aramon, unable to come to terms with his disgusting past.

In her bungalow on the disputed boundary, Audrun keeps herself and her meagre possessions spotlessly clean. But she cannot imagine breaking away to begin a new life, separated from the dramatic landscape she has known and loved all her life.

Tremain's intimate knowledge of the Cévenol is evident throughout Trespass. She evokes the coolness of the old dark house, its thick stone walls and high ceilings, reminiscent of a church roof. Anthony's excitement is contagious as he plans the restoration: "Restore the wood to its original colour. Re-plaster. Then tear the rendering off the walls and return them to stone. Dismantle the present. Get back to how everything had once been, and flood it with bright light."

Tremain summons up the stomach-churning experience of motoring as a foreigner in France: the treacherous mountain roads, the fear of misjudging a corner and crashing over the rock wall into the void. She deftly sketches the region's economic history: the decline of the silk industry, the toxic conditions of an underwear factory producing rayon girdles. But it is the sense of "wild nature", woods of holm oak, beech, chestnut and pine, with the river running through them and the threat of heavy rain hanging above, she captures so bewitchingly.

The story she weaves between her pair of siblings is taut and full of suspense that no reviewer should dispel. Suffice to say that the first chapter, which begins picturesquely, with a small child inspecting insects in the dusty grass at Mas Lunel, ends with a piercing scream that echoes through the rest of the novel until its gory denouement.

Tremain's present-day story wittily revives Robert Louis Stevenson's fears: perhaps foreigners still have good reason to arm themselves when they venture into the wilds of the Cévenol.

By Tremain's standards, this is a dark book, almost stripped of the humane optimism that characterised The Road Home, winner of the 2008 Orange Prize. Instead, Trespass evinces a steely grip on corrupt human nature, in all its ugliness and inadequacy. But there is, ultimately, redemption in a final scene that brings to mind the Lord's Prayer: "And forgive us our trespasses, as we forgive them that trespass against us."