In September 1992, an elite group of North Korean soldiers are put through a brutal training regime. Under the auspices of their commander, Park Mu-young, they will be sent into South Korea as sleeper agents, to be reactivated at a later date. The most promising of the group is Lee Bang-hee, a female sniper who assassinates several key South Korean figures over the next six years.
Over six years later, in September 1998, South Korea is hunting for Bang-hee. OP Center, a South Korean intelligence agency, is tasked with tracking her down. The agent in charge of her case, Yu Jung-won, has nightmares about her murdering both him and his partner, Lee Jang-gil. Jung-won is also engaged to a young woman, Lee Myung-hyun, a former alcoholic who runs a fish and aquarium supply store. Myung-hyun symbolically gives Jung-won a pair of kissing gourami, a species that cannot live without its mate, a gesture that foreshadows the intertwined fates of love, loyalty, and betrayal. Jung-won worries that he may not be able to tell her about the real nature of his job due to his security clearance.
Jung-won and Jang-gil are approached by an arms dealer who claims to have information about their quarry, but the dealer is shot dead by Bang-hee before he can reveal anything. Delving deeper, they discover the dealer had been contacted by the assassin to obtain something: CTX, a binary liquid explosive developed by the South Korean government. In its ground state, CTX is indistinguishable from water, but when heated under the right conditions, a small amount can trigger a devastating blast. The agents start to suspect its use when Kim, a scientist working at a lab connected to CTX, is killed by Bang-hee.
Mu-young and his North Korean operatives ambush a military convoy, seizing several liters of CTX and slaughtering the soldiers, escaping before the South Korean team can warn others. Jung-won and Jang-gil grow convinced there is a leak somewhere, as Bang-hee’s relentless precision suggests an insider. Bang-hee is ordered to eliminate the “kissing gourami” obstacle. Jung-won seeks outside assistance from Police Chief Ho, but Ho is accidentally caught in Bang-hee’s line of fire.
Mu-young taunts Jung-won with an ultimatum: he has concealed CTX bombs around Seoul and will give him a narrow window to locate each device, while also taunting Jung-won about his fiancée. A chilling memory surfaces: Mu-young once hijacked a civilian airliner, killing many, and escaped by disguising himself as a wounded crew member.
The first CTX bomb is found atop a department store, but Mu-young’s time-factor claim proves false. The device detonates just as the bomb disposal team identifies its exact location, causing numerous deaths. Jung-won and Myung-hyun seek safety in a hotel, and the stress drives Myung-hyun back toward alcohol.
Suspicions about Jang-gil as the leak intensify, with both men bugging the other to uncover the truth. A trap is set: Jung-won feeds Jang-gil misleading information, drawing Mu-young and his team into a deadly firefight that costs police and civilian lives. When Mu-young is cornered, Bang-hee intervenes to save him. Jung-won pursues a wounded Bang-hee, following a clue to a hidden aquarium light and discovering the bleeding Myung-hyun in disguise. He is left stunned, and proceeds to investigate her past relationship with the real Myung-hyun on his own.
Mu-young confronts Bang-hee about her hesitancy to kill Jung-won and reminds her of the mission’s primary objective. He calls OP Center, demanding millions and an escape plane for 2pm in exchange for the remaining CTX. Myung-hyun’s identity is confirmed by electronic surveillance devices found in fish décor throughout the OP Center, supplied by her. Jang-gil confronts her in her shop, and Mu-young shoots Jang-gil in a brutal continuation of the power struggle. Jung-won arrives with officers and engages Mu-young and his remaining agents in a chaotic firefight. Mu-young and Myung-hyun escape, while a dying Jang-gil hands Myung-hyun’s soccer match ticket to Jung-won, the event scheduled to start at 2pm.
The terrorists plot to detonate a CTX bomb directly over the Royal Box during an international friendly between the North and South, held at a stadium. Jung-won attempts to warn OP Center, but his involvement is dismissed. He races to the stadium, where Mu-young, Bang-hee, and others blend with the crowd. With liquids banned, they slip in as CTX and weapons pre-planted in the venue. Bang-hee retrieves a hidden Steyr AUG from a restroom, while Mu-young and his men shadow South Korean SWAT officers who retreat to the restrooms, where the policemen are killed and their bodies disposed of.
Myung-hyun advances toward the grandstand with her rifle, and Mu-young with his team, now disguised as SWAT officers, infiltrates the stadium’s control room. They force the staff to turn on the lights to trigger the CTX, while Jung-won arrives and notes the sudden glow. A tense confrontation erupts in the control room, resulting in the deaths of all four terrorists as the lights go out at the last moment. Myung-hyun fires at the VIPs but misses, and chases the retreating entourage through the stadium corridors, killing several more SWAT officers along the way before she is confronted by a large police force led by Jung-won. In a final stand, she is fatally shot by Jung-won as he closes in.
Jung-won later learns that Bang-hee (whom he believed to be Myung-hyun) was pregnant with his child and had left details of the renegade Northern agents’ plan and her intended location on his answering machine before heading to the stadium, asking him not to confront her directly and professing her love. Jung-won then seeks out the real Myung-hyun, who is in hospital undergoing chemotherapy. Myung-hyun reflects on Bang-hee with Jung-won as she listens to a song they once shared. Myung-hyun allows Jung-won to hear the lyrics, and as he does, he becomes deeply moved, the scene fading as he continues to listen with his eyes closed.
