Anyone in any doubt about the monumental task ahead of Barack Obama once he takes office next year would do well to watch the sobering documentary I.O.U.S.A.Unexpectedly entertaining, given that it’s about dodgy accounting on an epic scale, the film predicts an economic cold shower that is about to douse America’s prospects, thanks to its ever-expanding national debt.

The most serious threat to the United States is not someone hiding in a cave in Afghanistan, argues the former US Comp-troller General David Walker, “but our own fiscal irresponsibility”. He compares the current political and economic climate in America to the final days of the Roman Empire – and we all know how well that turned out.

There are similarities between this film and Al Gore’s environmental crusade, An Inconvenient Truth. Both documentaries follow impassioned and informed former government officials as they travel around the United States trying to raise awareness about their chosen cause. In this case, Walker rings alarm bells about the unsustainable and potentially disastrous fiscal policies of the US. Both films are almost, but not quite, scary enough to leave the audience running around blindly wearing buckets on their heads, screeching about the end of the world.

But while An Inconvenient Truth was essentially a PowerPoint presentation that relied on Al Gore’s political heft, I.O.U.S.A.works a little harder to prove its point. And let’s face it, it needs to. Despite the mind-boggling figures – at the time of filming the US had a federal debt of $8.7 trillion, but the figure topped $10 trillion in October – and the doomsday prophecies, economics is traditionally regarded as a bit of a snooze.

The director, Patrick Creadon, treads a delicate balance between keeping the audience awake and engaged, and leaving them sleepless and rigid with fear in bed at night. And don’t think, just because the debt has America’s name on it, that this doesn’t affect everyone. The message for us all is pretty bleak but it’s especially so for the incoming President.

As one grim-faced economics guru states: “We need presidents who are so devoted to doing the right thing with, and for, the American people that they are prepared to lose for their values.”

U, 84 minutes

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