Shiri was the first "Hollywood-style" big-budget action film to be produced in the "new" Korean film industry (i.e., after Korea's major economic boom in the Nineties). Created as a deliberate homage to the "high-octane" action cinema made popular by Hollywood through the Eighties, it also contains a story that draws on strong Korean national sentiment to fuel its drama. Much of the film's visual style is derived from the Asian action cinema of John Woo and Tsui Hark, but it is also clearly indebted to many of the films by American producer Jerry Bruckheimer.
The total budget of the film was only 8.5 million USD -- small by American standards, but at the time the single biggest budget yet allocated to a Korean movie (with part of the funding covered by Korean electronics giant Samsung). The film was a criticial and financial success in Korea and throughout the rest of Asia, and has since been issued on video worldwide. It has also played theatrically in limited engagements in the United States.
Warning: Wikipedia contains spoilers
An elite group of North Korean soldiers is put through a brutal training mission (which includes them murdering condemned North Korean prisoners with their bare hands). Under the auspices of their commander, Park Mu-young (Choi Min-sik), they will be sent into South Korea as "sleeper agents" to be reactivated at some later date. The most promising of the group is Lee Bang-hee, a female sniper who systematically kills several key South Korean government figures over the next few years.
Meanwhile, South Korea's OP Service is searching for Lee. The agent in charge of her case, Yu Jong-won (Han Suk-kyu) has nightmares about her murdering both him and his partner, Lee Jang-gil (Song Kang-ho). Yu is also currently engaged to a young woman, Myung-hyun (Kim Yoon-jin), the owner of a fish and aquarium supply store with a history of alcoholism. They are happy together, but Yu is worried that he cannot tell Myung-hyun about the real nature of his job due to his security clearances (as far as she knows, he has a bureaucratic desk job).
Yu and Lee are contacted by an arms dealer who claims to have information about their quarry, but he is shot dead before he can give them any information. After digging a bit deeper, they determine that he had been contacted by the assassin at some point, probably in an attempt to procure something. That something turns out to be "CTX," a binary liquid explosive developed by the South Korean government. In its ground state, CTX is indistinguishable from water, but when placed under the right temperature conditions for long enough, a single liter of it can explode with enough force to level a fifteen-story building.
Park and his commandos steal several liters of CTX as it's being transported. Later, Park calls Yu and issues an ultimatum: he has concealed several CTX bombs around Seoul, and will give him just enough time to find each one before setting them off. It is revealed that Park and Yu have a history: Park once hijacked an airplane and killed many civilians, but managed to escape by disguising himself as one of the (injured) flight crew.
The first of the CTX bombs is found on top of a department store, but Park has in fact lied about the time factor, and the bomb blast kills dozens of people. Yu is devastated, as is Myung-hyun, who has begun drinking again.
Yu realizes that there may be a leak within the OP, and suspects his partner -- who, ironically enough, suspected the same thing and bugged his car and phone to see if he could learn anything. They set a trap -- into which Park and the assassin step -- but after a horrific, bloody gunfight in public are unable to capture them.
It is then revealed that Myung-hyun is in fact the assassin, and that the information leak was in the form of fish being bugged with electronic surveillance devices. Yu himself had suggested bringing in the fish as a decoration for the OP office, and his girlfriend had supplied them.
The conclusion to the film is set at a soccer stadium, where both North and South Koreans are playing, unaware that Park has set a CTX bomb directly over the Royal Box in a light bank. Yu and Park tussle viciously, and Yu finally kills Park and turns off the power to the lights. Myung-hyun makes one last stand to finish her mission, but Yu kills her before she can do so. We learn that Myung-hyun was in fact pregnant with his child, and had left detailed instructions on how to catch her on his answering machine before leaving for the stadium.
The title of the film refers to a fish found in Korean fresh-water streams. At one point Park has a monologue wherein he describes how the waters from both North and South flow freely together, and how the fish can be found in either water without knowing which it belongs to. This ties into the film's ambitions to be the first major-release film to directly address the still-thorny issue of Korean reunification.
CTX is, coincidentally, also the name of a Korean electronics company.