“Flightplan” is two-thirds of a great movie and one-third of a mediocre one, but hey, even the mediocre one has Jodie Foster in it. She does movies so rarely these days that I keep forgetting how good she is, how her face can register so many different emotions at the same time, how she can take command of a scene with just her voice.

She’s especially good at portraying near-panic over the welfare of her children, as “Panic Room” and now “Flightplan” demonstrate, and I will join the chorus of people wondering why her first major role in three years is in a movie exactly like the last one.

This time she’s Kyle Pratt, a propulsion engineer working in Berlin who is flying back to New York with her young daughter Julia (Marlene Lawston) and a coffin containing her late husband David, who fell or leapt from a rooftop a week earlier. Mid-flight, Kyle wakes up from dozing to find Julia gone. Stewardesses help her look around the plane, but she’s nowhere to be found.

Panic starts to set in. Then the bomb drops: No one remembers ever seeing Julia at all. She’s shy and sad, Kyle explains. She didn’t talk to anyone, and she curled up in her seat as soon as they got onboard. Then it gets worse: The flight manifest doesn’t show Julia’s name. And then worse: But I won’t say any more on that, in case you haven’t seen the movie’s trailer, which tells more than it probably should.

This plot — child goes missing, but maybe the child never existed — has been done before, so we’re not giving points for originality. But I like how the screenplay (by newcomer Peter A. Dowling and “Volcano” scribe Billy Ray) handles the drama logically, at least for the first hour. Everything is laid out calmly and matter-of-factly: If Julia was ever onboard at all, she must still be. It’s an airplane; there are only so many places a person can go. (As it happens, Kyle helped design this plane, so she knows all the nooks and crannies.) On the other hand, if Julia never was onboard, then that means Kyle is crazy and possibly dangerous.

The crew, led by a sympathetic pilot (Sean Bean), is as helpful as you can expect them to be: first genuinely concerned about finding Julia, then sad to discover that Kyle is crazy, then annoyed that she keeps bothering them when there are beverages to be distributed. An air marshal who happens to be onboard (played by Peter Sarsgaard) must eventually make use of his handcuffs when Kyle simply will not take “your daughter was never here” for an answer, but not before she has disturbed all her fellow passengers and ignited fears of terrorism.

But what is at first a tightly wound psychological thriller eventually becomes something else. Directed by Robert Schwentke, this is a film where the questions are far better than the answers: Once we know what’s going on, we stop caring, because it turns out to involve one of those elaborate movie schemes that are so complicated and require such enormous foresight that you wonder why anyone would go to that kind of trouble.

Yes, sadly, the suspense of the first two acts gives way to a different kind of tension in the third, the kind where we wonder just how silly it’s going to get. (Answer: Very.) It’s not enough to ruin the picture, though, for even the absurd pyrotechnics of the climax and resolution are entertaining in their way. They’re not as smart as what led to them, but maybe you can’t have everything.

B (1 hr., 33 min.; PG-13, a smattering of violence, a couple profanities.)