Pure, By Timothy Mo

Just past the halfway mark in Timothy Mo's seventh novel, his heroine – a strapping Bangkok ladyboy who has joined a company of bloodthirsty Islamist warriors – wanders through a serene orchard on an island close to Singapore. Snooky's jihadi platoon (sanook = fun in Thai) has halted for a brief rest en route to help the insurgent Moros of the Philippines as fellow holy warriors. She (and for all the unwanted testosterone that jungle warfare brings, Snooky's chosen pronoun never wavers) samples the fruit of the ebony tree. It is not dark and dense like the wood, but pale, delicate and enigmatic: "tart and sweet in the same mouthful, soft but crunchy... If marzipan dormice grew on trees... they would taste like this". If this moment smacks of Eden, and forbidden fruit, it also helps initiate Snooky into the mysteries of the jihadi pursuit, where darkness yields to light, stark contradictions resolve themselves, and chaste perfection grows from the crooked timber of humanity.

For any major novelist to write the modern jihad from inside takes balls (which this heroine, in every sense, never lacks). But to make our first-person guide to the lofty idealism and flint-hearted cruelty that fuse in the zealots' mission a street-smart, razor-tongued Thai katoey calls on a degree of chutzpah that limits the field to just about one. Mo brings all his audacity and exuberance – frivolity, even – to one of the grimmest topics in our cultural lexicon. If Chris Morris's holy-war burlesque in Four Lions comes to mind, so do the baroque battlefield tragi-comedies of a Heller or a Waugh.

In the late 1980s, Mo broadened his range from poised Anglo-Chinese fictions such as Sour Sweet. After his Hong Kong historical novel, An Insular Possession, came an epic of guerrilla war in East Timor, The Redundancy of Courage. Then he went AWOL. Based in his native Hong Kong, Mo turned his back on the UK book machine and became a DIY self-publisher. He followed the on-off satire of Brownout on Breadfruit Boulevard with the rumbustious – and mesmerising - Filipino picaresque of Renegade or Halo2.

In Pure, an equally virtuosic HD performance, the knife-sharp but surgery-free Snooky has - courtesy of her English-language education - picked up a job as a film critic in Bangkok: a neat cover for Mo's cinema-buff riffs. Then a catastrophic drugs bust leaves her with Hobson's choice, delivered by a slimy Old Etonian top cop: 20 years in the Bangkok slammer, or a trip to her home turf in the disaffected Muslim south of Thailand to spy on old schoolfriends. They have enlisted in a religious school - the pondok - as camouflage for the local "boutique jihad". Since 2004, the insurgency in three of Thailand's Muslim provinces has cost more than 4500 lives; 16 died in attacks in Yala on 31 March.

Back among her own folk, who have "nothing, nix, in common with the Buddhist North", Snooky to some degree reverts to type. Although her masculine mask as the reformed "Ahmed" strikes her (and us) as skin- or beard-deep, the undercover agent finds herself becoming a Muslim again. Love, chaste and modest (quite a novelty for Snooky), does the trick: first for former school pal Jefri, now the purest of the pure, and then for their chief Shaykh, who enjoys his own passages of first-person narration.

This dashingly stern and severe warrior from Pakistan dreams of a new "Caliphate" that will stretch across South-east Asia. His upright devotion to the "purifying fire" of jihad blends his dad's Raj-era Indian Army ethos with a mathematical bent refined at Sheffield University. Shaykh despises the woolly way of thinking of "a Arts", with its blasphemous trap-door paradoxes, nuances and ironies (much like the book we are reading). His God is an abstraction grasped in "algebraic equations". And the formula for the Caliphate requires pharmaceutical doses of "anti-biotic violence". The group's attack on a disco in Phuket leaves 203 casualties. This carnage leaves Snooky feeling "scrubbed and alive... alive and clean".

Do we believe her? Not quite, for all that a doom-laden diagnosis has deepened her fatalism. Mo's problem, as Snooky joins the Band of Brothers (even Shaykh enjoys that Spielberg mini-series on DVD) in the Philippines, is that she never forsakes the spiritual glad-rags of the Bangkok movie theatre and night-club. Trying to act as godly "Ahmed" will always prove the biggest drag of all. And, as a storyteller, Mo prefers sanook to solemnity. So her adventures purchase terrific gusto and brio at some cost to credibility. Snooky, even as she hails a glimpse of mystic unity with fellow-soldiers as "the eye of my life", still mounts bizarre game-shows on video with hapless Western hostages – overwrought scenes, grotesquely mingling Swift and Tarantino.

We hear from another narrator: also a fervent idealist, although in a suave liberal-Anglican vein. Victor Veridian, Oxford historian, priest and veteran MI6 asset, comes out of retirement to "run" Snooky as an agent. Cue a delightful mimicry of the crusty donnish spook in the SCR, as gorgeously fashioned as all of Mo's voices and - it has to be said - as unstoppably garrulous as well. What does work a treat is this odd couple's internet fencing, as Snooky goes native among the jihadis. The web-mediated stand-off between this pair of feline sophisticates – comrades in camp, if nothing else - injects some salty to-and-fro into a novel that rests too often on dazzlingly single-minded monologue.

Shaykh has striven to make the world of his imaginary Caliphate "less complex", like a theorem from Euclid. Snooky yearns to shed the human artistry and multiplicity that she (in all ways) embodies in favour of "a moment of fulfilment, chaste and decent". That dream of cleanness will delude and destroy. Pure shows us why it still attracts.

Mo has talent to burn: as much soaring imagination, blistering wit, verbal felicity, narrative velocity and sheer take-no-prisoners bravado as anyone in his generation. Since he chose to fly solo, critic-proof and editor-averse, his task has been to find the right form to fit his tumbling ideas and galloping intrigues. Pure persists in that search for an ideal frame. Like the jihadis' kingdom of God, it proves elusive. But, for any open-minded reader, the sour-sweet flavour of its outlandish fruits will season and quicken the quest.