Infernal affairs abound in this dark thriller where corruption proves a unifying factor in the overlapping worlds of police, mobsters and financiers.

It is territory not unfamiliar to directing-scriptwriting duo Felix Chong and Alan Mak, authors of the Infernal Affairs trilogy, or producer Derek Yee Tung-shing, whose Protege entered a similarly dark universe. Add to the mixture three of Protege's stars, albeit in totally different kinds of roles, and Sean Lau, one of the screen's most celebrated character actors, and the ingredients are in place for an exhilarating yet thought-provoking celluloid ride.

The final result, although entertaining, is alas a case of unmatched expectations.

From its thematic concept to the cast and crew, the production possesses so much potential that its missed opportunities are all the more conspicuous.

Nothing could be more topical than Overheard's combination of the latest technology with a story based on stock manipulation and the havoc unleashed in its wake, both financial and moral. The expertise of the Criminal Intelligence Bureau's ace wiretap team of Johnny Leung (Lau, above centre, delivering the film's stand-out performance), Gene Yeung (Louis Koo, right, saddled with an unconvincing middle-aged makeover) and Max Lam (Daniel Wu, left) makes these cops privy to inside information and leads each to make life-altering judgment calls. Their off-duty existences are similarly tangled, not the least of which is Johnny's involvement with Mandy (Zhang Jingchu), the estranged wife of a colleague and friend (Alex Fong Chung-shun).

The opening reels of the mosaic-like narrative present a richly detailed and deftly edited introduction to the protagonists' jumbled private and public lives, interwoven with a suspenseful surveillance operation utilising the latest electronic gadgetry.

But the scenario gradually devolves into contrivances that are so patently manipulative that it becomes increasingly difficult for a viewer to remain blissfully immersed in the proceedings. A gratuitous plot device to justify the three stars' incessant cigarette consumption will leave tobacco companies smiling with delight.

Particularly unsatisfying is the grand finale, elaborately staged at the then-under-construction Stonecutters Bridge, but it's so far-fetched that further reflection raises more questions than answers.

On the other hand, the fact that one reflects on Overheard after leaving the cinema is proof that the movie contains both style and substance, even if not in the proportions required to turn Hong Kong's cinematic bears into bulls.

Overheard opens today