"Report back to me when it makes sense," the CIA boss tells an underling midway through the Coen brothers' deliriously confusing comedy thriller, which opened the Venice Festival last night.

Infidelity, divorce, murder, online dating, personal fitness, spying and cosmetic surgery are the ingredients in a plot which pulls in many directions at once.

Advance word had suggested that this was the Coens in more playful groove after the brooding and violent No Country For Old Men. The very title suggests something instantly disposable, a film to forget after seeing.

But the reason this shaggy dog story ultimately works so well is that it has a streak of darkness through it. If this is a goofy, screwball farce, it has a bleak undertow. The protagonists behave in random and often idiotic fashion but their actions have grim consequences.

The setting is Washington DC. Osborne Cox (John Malkovich in sneering, supercilious mode) is a mid-level intelligence analyst ousted from his job, partly because of his heavy drinking. His wife Katie (Tilda Swinton, more sneering and supercilious than Malkovich) is having an affair with Harry Pfarrer (George Clooney, looking as unkempt as he did in Syriana and Michael Clayton), a married federal marshal with a monstrous sex drive.

The Coens delight in showing the hypocrisy and duplicity of everyday existence in this little corner of upper middle-class, establishment America. The subterfuge which is part of Cox's professional life spills into the domestic sphere. Cox's world is contrasted with that of the employees of the Hardbodies Fitness Centre, a garish and soul-destroying place where Linda Litzke (Frances McDormand) and Chad Feldheimer (Brad Pitt) work.

The actors here offer variations on roles they have played. There is a hint of self-parody about their performances which is initially off-putting. At first, the film seems like a technical exercise, with the Coens allowing their cast to riff indulgently and play up to audiences' pre-conceptions about them. But the plotting is so embroiled that you are soon drawn into the drama and begin to forget about the affectations of the actors.

Everything starts simply, with Linda and Chad getting hold of Cox's memoirs, which are full of classified information, and trying to blackmail him. From this point, the complications begin to abound. One misunderstanding begets another until total chaos rules.

The Coens suggest this is simply a story about knuckleheads. But the film offers a damning portrait of a society in which the knuckleheads continue to hold the levers of power. The Coens introduce plenty of familiar, noirish elements. On one level, this is a dark thriller in which paranoia, avarice and sexual jealousy rule. But thanks to the increasingly absurd behaviour of the protagonists,it is also very funny.