“Non-Stop” opens Friday at Point, Eastgate, and Star Cinema. PG-13, 1:50, two and a half stars out of four.

Give Liam Neeson credit — he’s an action star who acts. In the “Taken” movies, “Unknown” and now “Non-Stop” (reteaming him with “Unknown” director Jaume Collet-Serra), Neeson manages to sell increasingly unbelievable premises with a world-weary gravitas and an edge of menace. In “Non-Stop,” he’s the captain, and even if we don’t know where we’re going and suspect the movie doesn’t either, we trust him to get us there.

In “Non-Stop,” Neeson plays Bill Marks, possibly the world’s worst federal air marshal. He’s emotionally ragged from personal tragedy and an alcoholic, seen in the opening scene taking nips from a bottle in the airport parking lot. And he’s afraid to fly!

But Bill is assigned to ride a transatlantic flight to London, and when he gets on the plane, it seems he’s been booked in Suspicious Glances Class. Everybody looks a little shifty-eyed, from the hardened New York cop (Corey Stoll) to the flight attendant (Michelle Dockery), from the pilot (Linus Roache) to the chatty passenger next to Bill (Julianne Moore). We’ll probably give the adorable little girl flying alone a pass, but you never know.

About midway over the Atlantic, the plot gears start to grind in motion. Bill starts receiving taunting threats on his cell phone from another passenger, who claims he’ll kill somebody on board every 20 minutes unless $150 million is wired to a special bank account.

How can you commit murder on a crowded plane? Our mysterious villain finds ways, and as the bodies mount, Bill grows increasingly desperate and paranoid, wrongly accusing and accosting one seemingly innocent passenger after another. Which plays right into the bad guy’s hands — on the ground, people are starting to think Bill is the real hijacker, in part because that bank account is in his name.

“Non-Stop” moves at the swift pace that it’s title suggests, but one nice surprise is that it’s much less an action movie than a locked-room mystery, expertly building suspense as Bill tries to puzzle out which person on the plane is the culprit. Although there is the occasional burst of action, including a terrific fight to the death inside a locked bathroom stall, brutal and almost intimate in the confined space.

But films like this do live and die on their solutions, and “Non-Stop” turns increasingly preposterous in its third act, both in its far-fetched plot revelations and in the CGI action finale we all expect is coming, in which our hangdog hero suddenly becomes almost superhuman.

Up until then, though, “Non-Stop” is decently diverting. I particularly enjoyed the teasing chemistry between Neeson and Moore, two fine actors who have done better than this together (“Chloe”), but don’t seem to mind elevating the material together. At one point, Moore talks about how much she loves flying overseas. “For six hours, nobody can reach you. You’re just living in the present.” The same could be said of enjoyable, disposable escapist entertainment like “Non-Stop.”