Robert Downey Jr., left, teams with Zach Galifianakis for the road-trip comedy Due Date. (Legendary Pictures/Warner Bros.)

Due Date reunites The Hangover’s breakout star, Zach Galifianakis, with director Todd Phillips for a sort-of update of John Hughes’s 1980s road-trip comedy Planes, Trains & Automobiles. Here, Robert Downey Jr. takes on the Steve Martin role as the uptight traveller trying to get home for an important event – in this case, the birth of his first child. Galifianakis, meanwhile, steps into the great John Candy’s plus-size shoes as the annoying shlub who joins him on the journey.

This is a road movie that starts running out of gas as soon as it hits the asphalt.

The Hughes picture was a tricked-out vehicle for Candy’s lovably obnoxious screen persona. Presumably, Due Date was meant to provide the same splashy showcase for Galifianakis. Instead, it’s a lemon. This is a road movie that starts running out of gas as soon as it hits the asphalt, a poorly navigated comic odyssey that too often leaves the ginger-bearded comedian spinning his wheels.

Downey plays Peter Highman, an anal-retentive architect preparing to fly from Atlanta back home to Los Angeles, where his wife, Sarah (Michelle Monaghan), is due to give birth by caesarean section. Galifianakis is Ethan Tremblay, a Hollywood-bound wannabe actor booked on the same flight. In the first of the movie’s many slack contrivances, Ethan’s attempts to get Peter to turn off his cellphone before take-off – while loudly using the words “terrorist” and “bomb.” It leads to them both being escorted off the plane by an air marshal.

Placed on the U.S. government’s “no-fly” list, and with his wallet stowed in the overhead compartment of his departed flight, Peter has no choice but to take a ride in Ethan’s rented Subaru. So begins their ill-starred cross-country journey, which has them zigzagging from the Mexican border to the Grand Canyon. Along the way, they score weed from a skanky single-mom dealer (Juliette Lewis), tangle with a lethal Western Union employee (Danny McBride) and become embroiled in obligatory car chases/crashes.

Most of these unscheduled detours and disasters are courtesy of Ethan. A mincing, scarf-draped drama queen-cum-pothead, he sashays about in acid-wash jeans as tight as sausage skins and the only male perm this side of the ’80s. He comes with a sidekick, a tiny French bulldog named Sonny, and a slavish devotion to the TV sitcom Two and a Half Men. When he isn’t causing calamity, his chatter shreds Peter’s tautly strung nerves like a dull cheese grater.

In Planes, Trains & Automobiles, the pathos doesn’t kick in until late in the movie, after we’ve had our full share of laughs. With Due Date, Phillips slathers on the schmaltz almost from the get-go. Ethan, who was in Atlanta to bury his beloved father, is just a lonely, well-meaning guy. “I have 90 friends on Facebook,” he boasts to Peter, before adding, “12 of them are pending.” (Cue collective sigh from audience.) Peter, whose own father abandoned him, is a combustible control freak who needs to chill out before he has a kid. Learning to deal with the child-like Ethan might be a start.

Due Date appears to suffer from one of those too-many-cooks screenplays. It was scripted by Alan R. Cohen and Alan Freedland, both vets of the animated series King of the Hill, plus Adam Sztykiel (Made of Honor), with Phillips also taking a writing credit. That may account for the patchy plotting and uneven tone. One minute we’re treated to masturbating dog jokes, the next we’re plunged into moody introspection as our fatherless duo glides down the interstate to the accompaniment of Neil Young’s Old Man.

Michelle Monaghan stars as a pregnant Sarah Highman in Due Date. (Legendary Pictures/Warner Bros.)

Downey, so witty and charming in his dramatic roles, is seriously unpleasant in his comic turn as Peter. His nasty interactions with the pot dealer’s towheaded kids (Jakob and Naiia Ulrich), as well as a scene where he spits on Ethan’s harmless dog, suggest a vicious if not emotionally disturbed man. Presumably, we’re supposed to feel some sympathy for Peter and his plight. Instead, this could be the actor’s most distasteful performance since Natural Born Killers.

Due Date is full of miscalculations like that, but its biggest crime is wasting Galifianakis. He’s the funniest thing in the movie, but only because he’s inherently comic. His character is a mess of contradictions and missed opportunities.

In one scene, we’re led to believe that Ethan is hopeless as an actor; in another, he does a silly but impressive impersonation of Marlon Brando in The Godfather. He prances about like a budding Corky St. Clair, yet the movie makes little use of his thespian pretensions. When he meets Peter’s longtime friend Darryl (Jamie Foxx), a handsome black ex-football player, he suspects him of being the real father of Peter and Sarah’s child – leading him to sow seeds of doubt in Peter’s mind. A smarter screenplay would have had fun with this reversal of the Othello scenario, with theatrical Ethan throwing himself into his role as a real-life Iago. Instead, the episode just seems to be there to show us that Peter is insecure and jealous as well as a jerk.

On Between Two Ferns, Galifianakis’s irreverent mock talk show for the Funny or Die website, he does a priceless interview with Mad Men’s Jon Hamm.

“You’re in The Day the Earth Stood Still,” Galifianakis says to him at one point.

“Yes,” Hamm replies.

To which Galifianakis simply responds, “Why?”

Now the tables are turned: Zach Galifianakis, you’re in Due Date. Why?

Due Date opens Nov. 5.

Martin Morrow writes about the arts for CBC News.