Probably the funniest exchange in The Big Short, a star-studded faux documentary/drama played for laughs, comes when a rabbi confronts one of Wall Street's most notorious figures - as a child. The grown-up Mark Baum, as played by Steve Carrell, is an investment cynic on an apparent mission to expose, destroy, and get rich from the system. In a flashback, we understand where he gets his campaigning drive.

"He's trying to find inconsistencies in the word of God", his exasperated Hebrew teacher tells his mother. "And is that bad?" she asks.

Carrell's character is one of the few real-life financiers in this film who has a moral compass, trying to unmask a system that is rotten and where inconsistencies encourage corruption - on a scale that astonishes even his world-weariness. And continues to astonish the rest of us.

The trouble is, while he may object to the ingredients and behaviour that led to the global financial crash of 2007, he also profits handsomely from it. As do all those involved in this thrilling, if sometimes confusing and repetitive analysis of how a few greedy fools hoodwinked a far greater number of even greedier fools within our financial institutions, busted a system that was destined to collapse by "shorting" the market, got away with it and then - along with all of the villains -slithered away to leave us with a gargantuan IOU.

Unusually, this is a film in which we all know the ugly, heartbreaking ending so the major thrill to be had is in understanding the beginnings. And this is where the film is strongest, especially in Christian Bale's Dr Michael Burry (he insists on reminding everyone of his medical credentials as if his actions are somehow protective). Part genius, part social misfit, he's perhaps the first to notice that the flood of ''bad'' mortgages is bound to swamp the market and leave enormous black holes in once cast-iron bank assets. His detailed analysis is ridiculed by everyone except Gucci-belted Wall Street wide boy, Jared Vennett, wonderfully played with oily arrogance by Ryan Gosling, who decides he can make a fortune out of exploiting the plight of people who are about to default on their mortgages.

He approaches Carrell with a once-in-a-lifetime business opportunity and we are suddenly thrust into a world of credit default swaps and collateralised debt obligations.

The use of recognisable stars - Brad Pitt turns up as Ben Rickert, an "ethical" trader who makes a fortune from the mess - helps an audience bemused by such terms to have some semblance of understanding of who's doing what to whom, and how.

The trouble is, none of it as gripping as Michael Lewis's book upon which this film is based; its revelations are not as astonishing as any of those in the myriad documentaries that have preceded it; it's much less dramatically involving than post-crash films such as 99 Homes and The Company Men.

And even though, as in the superior Margin Call, there's no one to root for, you can't help feeling The Big Short's director wants to imbue his leads with an undeserving heroism.

Yes, it's fun - as much as a film about ignorance, greed and apocalyptic hubris can be called fun - but it's a comedy short on laughter and a drama that misses the excitement of a denouement that we don't already know about.