Bong Joon-Ho's third feature, the amazing giant monster movie/intimate family drama The Host instantly shot him to the very top of the list of "South Korean Directors To Look Out For" - a list that has become altogether crowded in the last decade, making Bong's immediate fame that much more impressive. Three years later, and he's finally come out with a feature-length follow-up (after making the best short in the fairly dispiriting omnibus film Tokyo!), but anyone going into his new film Mother anticipating another film even remotely like The Host will be disappointed; no, not disappointed, because it's brilliant, but very surprised at any rate. Superficially, it's closest to Bong's sophomore effort Memories of Murder, in that it is at heart a detective story about the death of a young woman, but that's about as far as we can take it. Mother is its own animal entirely, and 14 films in, easily the best thing I've managed to see at this year's Chicago Film Festival, and I daresay the best thing Bong has made so far.

The film opens with a simple but somehow disconcerting image: a middle-aged woman pushes her way through a grassy field, and when she finally gets up close to the camera, she starts to dance to the music that just then pipes up on the soundtrack. We'll soon learn that her name is Yoon Hye-Ja (Kim Hye-Ja), and the exact context for what she's doing in that field becomes clear closer to the end (though it should be noted, she does not end up dancing when next we see her in that setting), but the shot isn't fascinating because it's part of some fractured plot scheme; there is nothing else in the movie that plays with chronology at all. The shot is fascinating because, well, it just is: the precise composition, the way that Hye-Ja stands out against the cloudy sky and the light grass, and of course the damn strange sight of a woman dancing for no reason in the middle of a field - it's just weirdly memorable, and haunting, and sets us up right from the start for a slightly off-kilter, otherworldly sort of movie. And that's pretty much exactly what we get.

Not to go into the plot too terribly far, because it is a mystery/thriller after all, Mother is about a single woman - Hye-Ja - with a grown son, Do-Joon (Won Bin) who is a bit weak in the head, and wholly dependent upon his mother to keep him from accidentally stapling himself to a doorjamb, or what have you. Their unconventional and slightly disturbing relationship - they share a bed, but since Do-Joon is evidently pre-sexual, it's not that bothersome - seems to be suiting them both just fine, except that, through a chain of events that are better seen than explained, Do-Joon ends up at the scene of a murder in one of the back alleys of the small town where the Yoons live. He manages to leave a fairly innocuous piece of physical evidence near the body, and is picked up in short order by the cops, who are perfectly delighted to let circumstantial evidence rule the day. Before you can say "reasonable doubt", Do-Joon is in prison, and Hye-Ja has decided to play amateur detective and clear her son's name by finding out who really killed that girl. Everyone in town knows her and her son, which makes her task both easier - she knows who to ask - and much, much harder.

Let us not mince words: like The Host this is, without a doubt, a much more audience-friendly movie than the bulk of Korean films that get released outside of Korea; Bong is not a filmmaker out to challenge and outrage us. But for all that Mother is a fairly glossy piece of cinema, it's also a profoundly effective one. Most of the movie's success rests on the depiction of Hye-Ja, and she does not let the movie down. For one thing, Kim's performance is absolutely magnificent, the kind of performance that seems to radiate straight out of the actress's body, so that every small hand gesture or eye movement contributes to our feeling that we're watching a real woman, really looking out for her son. The fact that she, like the rest of Mother's cast, is basically a cartoon character doesn't even really register, for Kim inhabits that cartoon with all the potency and emotional strength that the actor's art can bring to bear.

Bong and his cinematographer Hong Kyung-Pyo also do a fine job of composing the world around Hye-Ja, so that whenever she is in frame - and this is usually the case - she is the dominant object even if she's just a blip in a wide shot. The film is yoked to her point of view in a way that is absolutely necessary if the plot is to function (it's very important that we never be aware whether Do-Joon actually killed that girl, so that we like his mother must go solely on faith), yet the compositions not only serve that essential purpose, they also tend to make her a strong, centering presence; even in her moments of emotional anguish, she is the anchor that holds the film in place.

Here's the real bitch: no small part of what gives the movie its particular force is the ending, or rather the pair of endings, and how they grow from what else we've seen in the movie, and I'm not about to give that away; the success of The Host all but guarantees that Mother will get a US and European release, so just be patient. I will mention only the very last image, which begins as a fairly straightforward tracking shot, left to right, but quickly begins shaking and wobbling like an earthquake hit the camera rig, and so does the film descend into anarchy at the last moments, symbolising... damn! There's really just no way to discuss the ending without discussing the ending. Let me just lamely settle for arguing that Mother has a troubling, but very touching ending, and for all that the film is a carnival of grotesques, it also moved me very deeply with its unexpectedly humane core.