Secret Reunion

 At the turn of the millennium, a group of three North Korean agents led by the brutal "Shadow" (Jeon Gook-hwan) made an assassination attempt on a defected cousin of Kim Jong-il. The South Korean National Intelligence Service had turned one, Son Tae-soon (Yun Hee-seok), so Agent Lee Han-kyu (Song Kang-ho) and his team are able to get to the scene. It's a bloodbath, and Han-kyu is fired in the wake of the debacle. The other spy, Song Ji-woon (Kang Dong-won) is disavowed, cut off from his pregnant wife in the north. Six years pass, and Han-kyu is now working as a sort of very low-rent private detective, retrieving foreign-born brides who walk out on their husbands. Relying on his pair of idiot assistants gets him poised for a beating on a construction site, when the fight is broken up by the foreman - Ji-woon, living incognito under the the name Park Ki-joon. They recognize each other immediately, though don't show it, and soon Han-kyu has recruited Ji-woon to work for him. Each thinks the other's low circumstances is a cover, and that learning what is really going on will get them rewarded by their old masters.
The opening sequence of Secret Reunion is as good as this sort of spy movie stuff gets, fusing slick tradecraft with over-the-top action in a way that manages to evoke both the realistic and fantasy modes of spy cinema, set Shadow up as a villain to be reckoned with, and establish Ji-woon as a decent man without painting him as disloyal. Jang is so assured in this tense, violent territory that it's a bit of a surprise when the movie goes in a different direction.
Which it does; although the plot six years later is still a game of spy-vs-spy, it becomes something of a comedy, from the basic misunderstanding each has of the other's place in the world to their odd-couple antics: Han-kyu has become a slob and a bit of a joke, often bumbling around in ways that suggest he would have made a really terrible field agent, while "Ki-joon" is serious and compassionate. The movie takes us from gritty spy drama to what is frequently broad slapstick, but also lets the characters learn about each other as people rather than enemy agents. Jang manipulates the tone like a master, bringing whimsy and gravitas forward at the appropriate moments to prevent the movie from ever going too far in the direction of silly comedy or overwrought melodrama.
Of course, a lot of this work is from the actors, and the two main guys are brilliant. Song Kang-ho, well, you expect this from; it's just another in a long run of good performances in good movies from him. His disheveled charm fits this material like a glove, but he's not just putting on a familiar role; there's a sense of loss to it because we see him as an intense, forceful guy in the opening act, and there's always intelligence, if combined with wasted potential, to him even in his more goofball moments. Kang Dong-won plays off him nicely as the straight man much of the time, but also does well in projecting a guy clearly torn in many directions: He's a good man who hates violence but is still loyal to his country, and affection for Han-kyu does a nice job of sneaking into his portrayal. The pair never slip into practiced buddy-cop banter, but certainly grow closer.
If Secret Reunion has a weakness, it's perhaps in the end, as the spy material eventually grows more serious. A few logical holes appear, and while the film had never been perfectly realistic, it stretches the bounds it had established for itself in multiple directions. Not to the breaking point, but enough to be noticeable, especially when compared to how smooth things had been before.