Well, yes, eventually “Unknown” becomes the kind of movie where an amnesiac tough guy tells an enemy, “I didn’t forget everything. I remember how to kill you, a**hole.” Which is too bad, because for a while it’s not that kind of movie at all.

The psychological thriller, directed by Jaume Collet-Serra (“Orphan”), stars Liam Neeson as an American botanist named Martin Harris — or “DOCTOR Martin Harris,” as he keeps emphasizing. Dr. Martin Harris and his icy wife, Liz (January Jones), are in Berlin for a biotech conference when Martin is injured in an accident. He wakes up in a hospital four days later to find that no one in his life has any idea who he is.

Even more perplexing: Liz has a husband named Dr. Martin Harris, but it’s not him. It’s some other guy (played by Aidan Quinn). This guy claims all the same biographical details as our Martin, and Liz says she’s never seen the Liam Neeson Martin before. Has our Martin suffered a brain injury that’s giving him delusions about his identity? Is he who he really thinks he is? Is this an elaborate conspiracy to make him think he’s crazy? What, if anything, the hell is going on here?

To suddenly find yourself uncertain of something as essential as your identity must be nightmarish, and “Unknown” has a few moments that exploit the eeriness of it. The “Twilight Zone”-ish premise is compelling, Collet-Serra doesn’t let things get too slow, and Oliver Butcher and Stephen Cornwell’s screenplay (loosely adapted from Dider Van Cauwelaert’s novel “Out of My Head”) has Martin finding small answers often enough to stave off total exasperation, both for him and the viewer. He gets help from a Bosnian cab driver (Diane Kruger) who was with him at the time of the accident, and from an investigator — a former Stasi officer — played by Bruno Ganz. (Ganz played Hitler in “Downfall,” scenes from which have been used in many Internet parodies and mashups.)

It was smart to set the film in Berlin. (The book wasn’t.) This is a city that seems cold and forbidding to outsiders, a city that’s famous for having a split personality and that would like to forget much of its own past. There’s a faint whiff of Cold War mystique that adds to the unease inherent in Martin’s situation. You automatically get more tension by shooting in Berlin than you do in, say, Paris.

But over time the plot grows increasingly preposterous, morphing from an intriguing, “Memento”-style mystery into a more mundane action vehicle. The explanation for Martin’s predicament is clever; the way everything goes after we get the explanation, not so much. (And the less said about January Jones’ blank, emotionless performance, the better.) Neeson is always magnetic, though, even when he’s just repeating his “Taken” performance and delivering cheesy one-liners, as he is eventually required to do here.

B- (1 hr., 53 min.; PG-13, a little profanity, brief sexuality, moderate action-movie violence.)