The early chapters of Justin Cartwright's novel offer a brilliantly waspish portrait of a clever young man railing against life.

He has just been jilted by his girlfriend, Emily, a woman who had "a tendency to look at the world and see what aspects of it she could appropriate for her collection of useful spiritual truths". He descends into self-pity, a state of mind he prefers to describe as anomie, "which has redeeming associations with the artistic".

Richie is an Oxford graduate who was a first-class student, but has as yet to find a proper job. In a bid to escape London and memories of Emily, he heads to Jerusalem, with the help of a grant from his old college, to study medieval art in the region. The subject is not chosen at random. Richie's feckless but charming father, now dead, had been obsessed by the idea of following the trail of Richard the Lionheart and the tantalising hints he left that suggested he might have discovered the cross on which Jesus Christ died. And if that were so, where is it now?

Richie's explorations start slowly, not least because in Jerusalem he meets a woman who knocks Emily into the shade. Noor is a Canadian journalist, of Arab descent. Within weeks, in the least convincing and most perfunctory section of the novel, they are engaged. Shortly after she leaves on assignment to Egypt, and promptly disappears, just like the jewel her name evokes. Richie eventually learns she has been kidnapped, and that she may not be the entirely innocent journalist she first appeared.

So far, so very Richard the Crusader, who also languished in captivity, awaiting his release. Richie, however, is no warrior king and can do nothing to liberate his lover but sit by the phone.

Though Noor's predicament is to play a pivotal role, she scarcely merits the word cipher. Pawn would be more accurate, Cartwright using her mercilessly for his own ends as romantic interest and emblem of the age-long political mayhem and treachery of this inflamed part of the world.

Richie, meanwhile, grows up fast. While waiting for news of Noor he returns to London, where he digs deeper into the archives and stumbles on a document that might lead him to the cross. Interleaving chapters with passages from Richie's notebook describing Richard the Lionheart's adventures, Cartwright reveals his own fascination with this period. At first dry and factual, these sections grow increasingly vivid, as Richie's taste for the past is whetted, and his feeling for it grows.

Like anyone who tries to fathom a previous age, he has no illusions about bringing fresh wisdom to bear: "The more I learn about these times the more I find myself wondering how people managed to live in an age of fear, with the dark clouds of violent death, the plague, lawlessness always ready to rain thunderbolts on them. I doubt if it is fully possible to inhabit their minds. It is hard enough to understand the minds of others in your own time and in the same room."

Even so, these historical interludes are compelling; as, indeed, is the whole tale. Cartwright's research may be somewhat too evident in places, but this is offset by the infectious enthusiasm he conveys, and the convincing way in which he brings the crusading king to life.

Lion Heart is a highly ambitious book, the tangled connections between past events and modern players plaited with sophistication and an effortlessly beguiling style. The Richie we first meet soon evolves into a man of greater maturity and thoughtfulness, his academic research mirroring a personal odyssey - such a freighted word is deserved - that encompasses his long lost father, and his own unhappy childhood.

A Romance in the bygone, broadest sense, Lion Heart is not wholly successful, its plot too contrived, its reach a little too long. Yet even a less than perfect book by Cartwright is a pleasure, for the authority of his style, his intellectual mettle and his sentimental, courtly heart.