"Due Date," like its spiritual predecessor "The Hangover," is a weird emulsion of comedy and torture porn.

Human Muppet Zach Galifianakis is Ethan, an oblivious airhead with dreams of Hollywood acting glory. Master thespian Robert Downey Jr. is Peter, a high-strung businessman en route to L.A. for the birth of his first child. When they are forcibly de-planed in Atlanta, they share Ethan's rental car, with Peter being tormented at every turn.

A diagram of the film's plot would go like this: Stun gun, obnoxious kid, sexual impropriety, sexual impropriety involving dog, car wreck, broken arm, crematorium-remains coffee, psychological harassment, handcuffs, gunshot wound. These high jinks could go just as easily in a "Saw" or "Jackass" movie.

The distinction here is the cast. Downey plays the long-suffering Peter with a completely inappropriate degree of conviction. His carefully suppressed irritability comes leaking out, first in a steady trickle of sarcasm, then in geysers of retaliatory abuse. Ethan ruins Peter's life with full deniability; it's always an idiot mistake.

Peter's counterstrikes are premeditated. Yet Ethan, a stoner nincompoop, always forgives his new buddy. If you don't see that Peter is on the path to learning some life lessons, you have never seen a movie before.

For a road movie, "Due Date" generates hardly any momentum. Juliette Lewis and Danny McBride pop up, director Todd Phillips makes his compulsory cameo appearance, and Jamie Foxx zips in and out in a minimalist cameo as the movie slows to a crawl. The slack, episodic pace gives us time to reflect on the fact that we've seen almost every joke in the film elsewhere.

The pinnacle of the verbal humor comes when Peter asks Ethan why he carries his father's ashes with him in a coffee can. "Because he's dead," comes the reply.

By reuniting with "Hangover's" star Galifianakis, Phillips may have thought he could recapture the magic. Instead, he created a low-grade migraine.

Colin Covert • 612-673-7186