If there is one great huge problem with Forgotten, and I rather feel like "problem" is a bit of a leading word, it's that the film is nakedly, overwhelmingly anxious to remind you of Oldboy. And surely, Oldboy is a wonderful movie, so that's an understandable impulse, but we're still stuck with a film that, for all its appeal, never quite manages to escape a rut of scenes that trigger a "yes, but I saw that in Oldboy, and it was better" reaction. Not every scene, nor the overall plot. Also Forgotten isn't an action film, nor a revenge thriller (or, at least, not anything resembling the same kind of revenge thriller). It's more of a shared vibe, and a structure of shocking! twists! that works in much the same way and much the same rhythm as the 2003 film. But this is the last I will talk of any of this.

Because, for all its limitations and shallowness, Forgotten is a hell of a movie: a little sick, a little sad, and a lot confusing and mysterious in all the right ways. It's one of those movies that makes one a little bit upset that it's streaming-only (outside of its native South Korea, it's a Netflix exclusive worldwide), because it feels like it would be a real hoot to watch it with an appreciative audience; it's also one of those movies that would sink without a trace if it had to compete in a crowded theatrical marketplace, so it's just as well that we can get it in some format.

The film opens with an abrupt jolt: teenage Jin-seok (Kang Ha-neul) bolts out of confused, vaguely violent dreams in the backseat of a car, next to his mother (Na Young-hee), behind his father (Moon Sung-keun) and older brother Yoo-seok (Kim Mu-yeol). The family is on their way to a new house, and upon arriving, Jin-seok has a striking sense of déjà vu; it's not the last odd thing to happen by any means, as the young man finds a door that his father insists, with curious intensity, is never, ever to be opened. And even that's not much compared to the wet night when Yoo-seok is kidnapped. That's bad enough, but it somehow gets worse when he's inexplicably returned 19 days later, without a word from the kidnappers: Yoo-seok has no memory of what happened, and the brothers' parents don't seem any more interested than he is to find out what did. Thus it falls to Jin-seok to solve a mystery that might not even be one.

That's already going a little bit farther into the story than might be necessarily desirable - it's a film that wants to be seen blind - but Forgotten has a particularly difficult story to write about, even as the fascinating irregularities of the story are the most significant thing about it. Not entirely "significant" for good reasons. Let us say, without going into it, that something happens close to midway point that requires us to substantially re-evaluate what the movie has been to that point. And this is a heady, exciting thing. At the same time, the nature of that re-evaluation makes it really damn hard to maintain emotional investment in the characters and story.

The film that results is a bit robotic: it sure as hell works mechanically, on this viewer at least, prodding and pounding away, yanking my expectations and feelings in different directions to dizzying effect. What it does not do is yoke this particularly well to the development of its central character, and this becomes somewhat conspicuously the case as the second half goes on: it's as much a psychoanalytic study of Jin-seok as it is a mystery-thriller, but firm limits on how much it can tell us about him have been burned into the basic scenario. It also becomes pretty complicated for little clear reason other than the pleasure of complexity.

None of which is to say that the film isn't a blast to watch. Writer-director Zhang Hang-jun, making only his third film (and first since the 2003 romantic comedy Spring Breeze), has learned all the right lessons from watching other thrillers - a strong genre for the South Korean film industry during the 21st Century - and if Forgotten ends up feeling a bit like a mélange of influences rather than a standalone work of creativity in its own right, it is a superb mélange. Given the extreme U-turns in tone that the film makes, and an even more extreme adjustment to its POV, it must have been hell to leave this feeling like it flowed smoothly and inevitably from scene to scene, but Zhang has left no visible seams in the construction of the story, and even though I feel like you could pinpoint almost to the exact frame where the big swerve happens, there's no sense, in watching, that the momentum falters or hangs. Indeed, momentum is one of the film's greatest assets: it shifts from a gentle, sunny family story in its opening scenes to a mildly menacing thriller with almost imperceptible slowness, becomes a psychologically disordered mystery with a bit more speed and force, and only keeps picking up speed and urgency as it enters its second half.

The film is, if nothing else, exciting to watch: while I suppose it is possible to get out in front of the story, I think it would take an unusual genius to do so, and the steady drip of surprises makes it easy to remain invested. Kang is charismatic as hell, readily able to tap into the boyish enthusiasm of playing amateur detective, and that helps to make Jin-seok an eminently satisfying protagonist, even above the incipient interest that the basic concept generates. Once the plot starts to go into its stranger corners, Kang's mere presence is sometimes the only thing keeping it all stapled together - this seems more like a deliberately strategy than a sign of poor filmmaking - and provides the film with some kind of throughline that it otherwise conspicuously lacks.

At any rate, Forgotten is something like the platonic ideal of a Netflix film: easy get swept up in, challenging without being actually hard, not terribly concerned with re-watchability or sticking power. Better thrillers come out all the time, and yet I find that this has almost everything I want out of a thriller: it moves fast, it never runs out of dark corners to explore, it has strong personal stakes, even if they're not, by the end, the same as the stakes it started out with. It's just darn solid moviemaking, satisfying and stylish (the cinematography is full of suggestive shadows, particularly in the nighttime exteriors), and clever enough that it can get by without being too smart.