When his flight plans go awry, an uptight businessman with a deadline on making it home to his wife ends up sharing a ride cross-country with a quirky, obnoxiously over-friendly fellow traveler whose cheerful demeanor shields a tragic past. This is the basic plot description for "Due Date," director Todd Phillips' follow-up to 2009's rapturously overrated "The Hangover," but it could just as easily describe 1987's alternately hilarious, heartbreaking "Planes, Trains and Automobiles." There's really no way of avoiding the comparison since these two films are so similar; indeed, "Due Date" could actually be considered a loose remake if the makers actually copped to stealing wholesale from that John Hughes classic. Whereas "Planes, Trains and Automobiles" stands tall as one of the great comedies of the past quarter-century, able to make the viewer guffaw with laughter one minute and tear up the next no matter if he or she has seen it once or a hundred times, "Due Date" comes off as a pale, uninspired rendition that will be all but forgotten in a few years' time.

With pregnant wife Sarah (Michelle Monaghan) scheduled for a C-section in three days, architect Peter Highman (Robert Downey Jr.) is looking forward to returning home to Los Angeles after finishing up a business trip in Atlanta. When an altercation on his plane with uncouth fellow passenger Ethan Tremblay (Zach Galifianakis) gets both of them kicked off and placed on the no-fly list, Peter, whose wallet and all his cash have gone missing, has little choice but to agree to accompany Ethan back west. Wacky misadventures ensue, with Peter at first barely able to tolerate Ethan and then growing to sympathize with his plight (coming from his father's funeral, Ethan wants to stop off at the Grand Canyon to scatter his ashes).

No fault of Zach Galifianakis (2010's "It's Kind of a Funny Story"), who exhibits on-target comic timing and the ability to also give his character a certain amount of pathos, but Ethan Tremblay is every bit as annoying as Peter thinks he is. One feels sorry for the lonely Ethan, whose best friend is his pug and who has overzealous dreams of going to Hollywood and making it as an actor, but that doesn't make him any more tolerable. Socially inept, Ethan blatantly masturbates before bed when the two of them have to sleep in their rental car and inconsiderately laughs at Peter when he tells him a sad story about his own father walking out on his family when he was six. Peter's frustrations over his travelmate are understandable, but he's not exactly the nicest of guys, either, physically and verbally berating Ethan, spitting on his dog, and, in another scene, intentionally running over drainage ditches while Ethan rides in the back of a pick-up truck. Whereas the deep affection and understanding that accumulates between odd couple Steve Martin and John Candy in "Planes, Trains and Automobiles" is increasingly evident, the friendship between Peter and Ethan comes too late in the game and feels forced by the requirements of the formulaic screenplay credited to four—count 'em, four—writers. Alan R. Cohen, Alan Freedland, Adam Sztykiel and Todd Phillips owe a lot to that earlier picture, yet have missed the mark in trying to replicate its genuine humor and ultimate humanity.

Robert Downey Jr. (2010's "Iron Man 2") is effective as straight-man Peter Highman, performing as written but not allowed to develop his role much beyond a surface level. A subplot in which he suspects his wife cheated on him with mutual friend Darryl (Jamie Foxx) is dropped almost as quickly as it's introduced. As Ethan, Zach Galifianakis creates a person who actually comes off as authentic in his offbeat personality, but not someone you'd necessarily want to spend time with. When Ethan reveals something that he has been hiding from Peter all along, it is supposed to make the viewer realize how desperate he is for a companion, but only really serves to cement how selfish he is. In relatively thankless secondary roles, Michelle Monaghan (2008's "Eagle Eye"), as Sarah, spends most of her screen time talking over a phone; Jamie Foxx (2010's "Valentine's Day") awkwardly shows up and exits with little impact as Darryl, and Juliette Lewis (2010's "Conviction") continues to be typecast as drug dealer Heidi, whose house Ethan makes a pit stop at to pick up illegal provisions.

With most of the comedic highlights (and payoffs) given away in the trailers and television ads and the rest just embarrassing (a scene where Peter insults an Iraq war vet played by Danny McBride and is accosted by the paralyzed man is painfully tacky), "Due Date" is never as funny as it hopes to be, nor as emotional as its sporadic serious moments intend. One can guess where the plot is headed at every step, but, even if we don't believe it, the film is still paced well enough that it never becomes an all-out burden to watch. With a couple small laughs, a few smiles, and a strong soundtrack nicely mixing the old with the new, "Due Date" diverts disaster even as it wallows in mediocrity. It's too safe to go anywhere that isn't predictable and not clever enough to reach the raucous high it wants to.