“Non-Stop” is a sky-high downer.

What a waste of a great premise: a locked-room mystery at 30,000 feet. Liam Neeson plays an air marshal on a New York-to-London flight. Somewhere among the 150 passengers and crew is an extortionist threatening to kill innocent travelers one by one unless a large ransom is paid. How can the lawman thwart the plot? How can he even investigate without throwing the cabin into mass panic?

The gloomy prologue had me leaning forward in full upright position. The opening scenes, with Neeson pounding down a pre-flight cocktail of Scotch and breath spray, tersely peg him as a seriously flawed hero. Director Jaume Collet-Serra shoots the introduction in shallow depth of field, echoing Neeson’s woozy perspective.

We observe fateful details. Who is the fellow who knowingly offers Neeson eyedrops in the airport lavatory? Why does Neeson’s seatmate on the flight, Julianne Moore, have a surgical scar on her sternum? Rising star Lupita Nyong’o (“12 Years a Slave”) plays a seemingly minor role as an air hostess. Is that just a quirk of casting, or is she a key character in camouflage? The suspense fan in me was more than ready for takeoff.

Then the film tailspins from whodunit to whydidtheybother? Bodies start dropping. Taunting text messages appear on Neeson’s cellphone. Implausibilities pile up like luggage on a baggage claim carousel. The extortionist has an almost telepathic knowledge of Neeson’s background and minute-to-minute actions, and is a deadly sniper with a peashooter. Characters change their stripes from belligerent to heroic at the screenwriters’ whim. A near-mutiny among terrified fliers is quashed with a promise of free air travel.

Nothing stacks up, as the movie wallows in “Snakes on a Plane” absurdity. Logic is the first casualty in contrived thrillers, and this script, by an undistinguished committee of writers, murders it with a thousand wounds.

Physics takes a pounding, too. Once again we see a gunshot through the fuselage creating a catastrophic vacuum vortex that rips the plane’s hull apart. That’s impossible. The air pressurization system would compensate for a bullet-hole leak. It wouldn’t even ruffle your hair. And don’t even get me started about the absurd power-dive episode of zero gravity.

What a shame it is that Neeson’s best role in years is as a teensy plastic policeman in “The Lego Movie.” He’s a powerful presence, both in his athletic assurance (no creaky joints for this 61-year-old) and his mournful gravitas. Yet he’s becoming a synonym for mindless action-movie mediocrity.

From the A list to “The A-Team.” Now that’s a rough landing.