Film review: The Hangover Part III - it tries hard to be funny but fails to raise a solitary guffaw despite Zach Galifianakis' borderline sociopath Alan

Before the lights went down at the press show of The Hangover Part III the PR person told the assembled it was "worth staying for the credits at the end".

She omitted to mention whether it was worth staying for the actual movie. It's said that tourists from all over the world now go to the front desk at Caesar's Palace in Las Vegas and ask – in tribute to Alan's gormless enquiry from the first film – "Is this the real Caesar's Palace?"

It was a gag that came beautifully out of nowhere, just like the movie itself. The success of The Hangover took everyone by surprise, which is the one element that its makers simply can't duplicate – though God knows they tried, transferring the bachelor mayhem to Bangkok for The Hangover Part II. Which is how it ended up looking like a cheap Eastern knockoff.

This third and (we are assured) final part is at least an improvement on Part II. Within five minutes there has been a decapitated giraffe and a freeway pile-up, both naturally the handiwork of Alan (Zach Galifianakis), borderline sociopath and cause of the original Hangover. Alan's mother in despair asks his pals Phil (Bradley Cooper) and Stu (Ed Helms) to take her errant son to a cure home, even though they realise that nothing short of death is going to cure Alan.

And they've no sooner hit the road than they're waylaid by a mob boss (John Goodman), who wants them to track down their old nemesis Chow (Ken Jeong) and the stash of gold bricks he stole. The gangster takes Alan's brother-in-law Doug as ransom, which means third time unlucky for Justin Bartha, disappearing on cue.

Director Phillips has promised to go for a "truly epic finish", and with co-writer Craig Mazin cooks up a plot that stays just about roadworthy. We get a trip down Mexico way, a break-in at a mansion, an arrest, and thence a return to Vegas, scene of the original crime. It's a curious thing to watch, because all the comic architecture is in place, the rhythms of comic inter- play are at work, the lines are delivered with a comic relish – and yet almost nothing in it is funny. The one time I heard genuine laughter ripple over the stalls was Stu dry-heaving at some inanity of Alan's.

Whereas the "Wolfpack" trio made nearly everything they said sound hilarious, now they seem to be waiting around for each other to raise a laugh; Cooper and Helms are reduced to mugging for the camera, offering reaction shots to nothing. Galifianakis, the original's wild card, has the saddest moment of all, showing puzzlement at an architect's model of the house they're planning to burgle – and revisiting the Caesar's Palace gag. Oh no! I think everyone stayed for the closing credits, as the PR had advised, and, a few titters aside, the auditorium remained as quiet as a church.