Fiction is, by its nature, fuelled by what-if scenarios, some more commonplace than others. Nicholas Shakespeare's novel deals with one that must be almost universal: the question of what would happen if one became a multimillionaire overnight. Beyond the inevitable fantasies of material extravagance lies the alluring belief that infinite wealth would enable you to become the person you were always meant to be - a delusion that no doubt helped the National Lottery's total ticket sales reach over £5m in the year to April 2009.

There seems no doubt that an infusion of cash would substantially improve the life of Andy Larkham, a hapless, likeable editor of bargain-basement self-help books (sample title: Learn to Make Your Black Dog Your Guide Dog). We first encounter him borrowing money from a co-worker. Later, he is painfully dumped by his fiancée, who is a model, for - or so he suspects - being too poor. So when he dashes accidentally into the wrong funeral (having borrowed a twenty for the cab), he is overwhelmed to learn that the mistake has earned him an inheritance of £17m.

This is an intriguing premise, and at first Inheritance seems to mine away at the money-won't-buy-you-happiness seam that one might have thought F Scott Fitzgerald had thoroughly exhausted. Andy furnishes himself with a personal wealth manager, a Mercedes Cabriolet and a flat in Kensington, and is so hurt when friends fail to appreciate the skill with which he has chosen his new fireplace that he embarks on a grand tour of Europe's casinos, cigar conventions and Bugatti rallies in the company of an unmemorable slew of beautiful girls.

So far, so unedifying; but Andy, who is periodically haunted by memories of a teacher obsessed with Montaigne, is not the sort of chap to live on lotuses for ever. Inspired by a visit to the philosopher's castle, he decides to find out the identity of his dead benefactor, a decision that entails plunging into another story altogether. These two elements, the first a 21st-century comedy of manners and the second a sober account of an Armenian exile's troubled life, are, despite their evident links, inadequately fused, and are so different in tone and impact that they seem to belong in different books.

The story of Christopher Madigan, a mining millionaire whose life was beset by personal and political catastrophe, is spare, finely written and genuinely moving. But the tale that encases it seems by contrast emotionally underpowered and a little silly. The publishing in-jokes don't always fly and there are too many unlikely events that require a touch of gloss if we are to swallow them.

When Andy is dumped at his favourite Portuguese restaurant, it transpires that his girlfriend has brought her new lover along to witness the scene. This is an unusual scenario, and might have been more easily accepted if Andy had expressed surprise, at least with a passing exclamation. The greater unlikelihood of the inheritance is better managed, the author offering an exhaustive explanation of the legal ramifications of handing over your entire fortune to whatever random friends or strangers happen to attend your funeral.

The graver problem concerns what might in a different era have been described as the novel's moral. It sounds faintly passé to talk of morals now, but in a text explicitly guided by Montaigne and filled with such homilies as "Luxury in its ravages is more terrible than war", it is difficult to ignore the matter.

It's evidently intended that the reader should consider Andy's trajectory as arcing healthily towards maturity of an emotional, even spiritual, kind. He outgrows his pleasure in vulgar luxuries and enters a life of what is apparently to be regarded as soulful work: namely, listening to Christopher's former housekeeper recount his life story while quaffing an awful lot of good red wine (the bottles are swooningly indexed: a 1982 Pétrus, a 1997 Sammarco). This accomplished, "he had the opportunity to achieve something" by rehabilitating Christopher in the eyes of his estranged daughter.

As for his own rehabilitation, its depth may be measured by the way he donates his Le Corbusier chair and his Warhol to charity with the sanctimonious explanation that "they belonged to another person, a friend he had had for a while and tired of without even the need for an argument". There is nothing wrong with tiring of Warhols, but it perhaps escapes Shakespeare's attention that Andy is simply exchanging his nouveau riche trappings for material comforts of a more patrician kind, trading the tacky flat for his benefactor's own pleasantly understated house, complete with wine cellar and garden tower, and his work as an editor for the undemanding, albeit "intense", life of a gentleman scribbler.

This lack of weight is only emphasised by the intensity and heft of Christopher's "trials". As with one of Fitzgerald's doomed heroes, his wealth brought him a fate very close to damnation. For the larky Larkham, on the other hand, money yields fulfilment on every level. It's an uneasy moral, but one that's appropriate for a fable of our times.

Nicholas Shakespeare
Harvill Secker, 272pp, £12.99

This article first appeared in the 19 July 2010 issue of the New Statesman, Godless Britain