Unlike the first reviewer of this movie here on IMDb, I found Secret/Sikeurit (a 2009 Korean movie) to be quite entertaining, and solid enough to recommend it as a worthwhile watch.
SPOILERS AHEAD!
This movie is categorized as a thriller; a genre that often encompasses a very broad range of films. More specifically, "Secret" falls into the sub category of being a Suspense/Mystery/Crime/Drama film. There's little to no action involved here save for the penultimate climatic scene, and that scene is the by far the worst part of the movie in my opinion. It's the 'before and after' the climax where this movie clicks, and often clicks quite well.
What we basically have here story wise is as follows: A detective is investigating a murder in which clues to the case continually lead to his semi-estranged wife. It's a fairly formulaic and straight forward premise at its core: Is wife guilty?...Should detective protect wife if so?...Will the outcome result in wife and hubby reconciling?...Is there something else going on that isn't quite apparent?...etc...
I thought the story, script, and plot of this movie were all relatively strong overall. The film did keep me watching, and I was interested enough in the mystery and eventual outcome throughout. It was fairly well paced as well, and was accorded solid performances by the leads throughout.
Overall, I very much enjoyed this movie. My primary issue with it is the climactic action scene that seemed to be completely out of tone with the rest of the film. This scene really should not have been included in the film at all, or, it should have at least been more restrained in order to result in a more convincing outcome. However, I'm fairly confident the director's intent for including this scene was to actually provide the viewer a "red herring" of sorts, and it was certainly effective in accomplishing that. Red herring or not, this scene definitely could have been tamed down quite a bit, or reworked in another equally effective fashion to better accompany the film as a whole. I feel this is a movie that would have been much better served by having no action scenes in it whatsoever.
Bottom line: I'm giving this film a 7 for the rating that will accompany this review.
P.S. FYI; As is becoming more prevalent these days in cinema from across the globe; it's VERY important to NOT stop watching this movie until the end, AND, until after the credits have rolled!