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What kind of events lead to long-lasting bro-bonding?

Friendship -- let alone strong bonds -- hardly seems possible at the start of "Due Date," when Robert Downey Jr.'s face gets pumelled in an airplane aisle by Zack Galifianakis' doughy paunch.

It's a cringe-worthy moment to be sure, as Galifianakis struggles to stow some luggage above Downey's head.

So begins Todd Phillips' follow-up to "The Hangover," the highest-grossing R-rated comedy of all time. And the gross-outs and cringe-moments just keep coming.

"Due Date" won't go down in history as the best buddy road-trip movie ever made. Its stars will never share the same, illustrious gleam as Laurel and Hardy or Oscar Madison and Felix Unger.

But Downey and Galifianakis will make you laugh. That I guarantee.

Downey plays architect Peter Highman, a very harried, very serious hipster who is rushing home for the birth of his first child.

Galifianakis is cast as Ethan Tremblay, a would-be actor with tight jeans, a bad perm, and a persnickety attitude.

Thanks to a fateful brush with this boob, Peter is shot with a rubber bullet by an air marshal and tossed onto America's no-fly list. He loses all his belongings, money and dignity.

The only way this unlucky dad-to-be can get home is by accepting a cross-country ride from this pot-smoking airhead.

Trapped in hell, or in this case a Subaru Impreza, Peter endures hours of "get to know ya" questions.

"When did you lose your virginity?" Ethan asks, among countless other wildly intrusive and inappropriate questions.

Before Peter can roll his eyes, Ethan reveals that he lost his virginity at age nine. He has 90 friends on Facebook. He's 23 (in his dreams). And the TV show "Two and a Half Men" is his inspiration, but just season two. This actor has his standards.

This deluded little pixie drags Peter hundreds of miles off course so he can buy some dope for his "glaucoma."

Peter endures car wrecks, ill-omened border crossings into Mexico and a cup of coffee accidentally brewed from the ashes of Ethan's dead dad.

Yet even as Peter and moviegoers stare aghast at this fruitcake, Ethan's humanity wins our affection and a ticket into Peter's heart. This lonely little screwball misses his late father. He longs for someone, anyone, to talk to so he won't feel all alone in this world.

In ways that defy all reason, cockamamie Ethan understands that Peter is not quite ready for fatherhood.

He sees past Peter's cool, handsome exterior and recognizes that he secretly worries about his wife's fidelity and if his new child is really his.

"Due Date" may not have the most brilliant plot development. It is also a bonanza of bad taste, so if Galifianakis' raunchy humour is not your bag, stay far away from this one. But when push comes to shove the fat man saves the day in this crazy romp. And Downey's slow, beautiful burn through hell makes this ride well worth taking.

Three stars out of four.