For two smart, nerve-wracking acts, "Flightplan" is a thriller almost worthy of the tag "Hitchcockian," in which Jodie Foster plays a distraught mother whose forlorn 6-year-old girl has disappeared in the middle of an overnight flight from Berlin to New York.

Already an emotional wreck because her husband has just died -- his coffin is in the cargo hold -- when Kyle Pratt (Foster) wakes up three hours into the flight to discover her daughter gone from her side, she loses it. Frantically searching the state-of-the-art jumbo jet, she becomes so unruly that the passengers are put on edge, the captain is called, and an air marshal (Peter Sarsgaard) takes her into custody while the crew looks for the missing child.

But startling revelations soon emerge about the death of Kyle's husband and other seemingly indisputable plot particulars. The whole dynamic of the film, and your perception of this grief-stricken widow, soon shift wildly -- and more than once -- as director Robert Schwentke (a German making his Hollywood debut) deftly rolls mood, pacing and Foster's gut-wrenching, cracked-psyche performance into an atmosphere of incendiary tension.

Unfortunately, if you've seen the movie's trailer or TV ads, you've already been exposed to dumbed-down versions of these powerful twists in a way that ruins every good jolt the movie has to offer. At the point in the story where these previews leave off, "Flightplan" dives into a B-movie tailspin from which it simply cannot recover.

The third act kicks off with another big twist -- but this time it's so arduous, absurd and out of character for the film that the entire plot, stretching back to Scene One, is sabotaged. In an instant, the film disintegrates into unrestrained nonsense dependent upon an intelligence-insulting combination of dumb luck and laughably convoluted complexity.

Soon Foster's intense performance of raw emotion and redoubtable maternal instinct is all that's left keeping "Flightplan" in the air, as she sneaks around in the bowels of the plane, which she had once helped design in her job as a propulsion engineer. Even Sarsgaard, known for his understated potency in films like "Shattered Glass" and "Kinsey," seems overtaxed yet indifferent toward what turns out to be a disappointingly generic yet entirely preposterous role.

Employing skillful manipulation of anxiety and apprehension, director Schwentke does wonders with the first hour of "Flightplan," using cinematic slight of hand to almost subliminally plant the seeds of several cunning twists in advance, making each one seem all the more real and definitive. He even does a cautious but sharp-edged job of playing on cultural fears in post-9/11 air travel.

But once the script flies into the thin-aired realm of the ridiculous, no amount of talent behind or in front of the camera can bring this film in for a safe landing.