Plot summary

In 2000, an American military pathologist (Scott Wilson) is commanding a reluctant Korean assistant to violate protocol by dumping over 200 bottles of formaldehyde down the drain, leading into the Han River. Over the next few years, there are sightings of a strange amphibious creature in the water ways.Six years later, in Seoul, Park Gang-du (Song Kang-ho) has fallen asleep while working at small snack-bar. His father, Hee-bong (Byeon Hee-bong), chases them away. Gang-du wakes up thinking he heard the voice of his daughter Hyun-seo (Go Ah-sung), but it was a different little girl. Hee-bong gives Gang-du an order to deliver to some customers, but Gang-du eats part of his customers' food. Also with him is his brother and sister; national medalist archer, Nam-joo (Bae Doona), and alcoholic former-activist Nam-il (Park Hae-il).While Gang-du is delivering food to some customers, he sees a crowd along the Han River. They witness a huge creature hanging from the Wonhyo Bridge, which then jumps into the water. At first, it seems as though the creature has swam away, encouraging the public to bait it back with food. Moments later, the creature rises out of the river and runs amok. After the creature causes chaos and kills a number of people, Gang-du and an American man attempt to kill the creature with a metal pole, but in the process the American is seriously injured and they only succeed to anger it and starts pursuing Gang-du. Gang-du grabs a girl's hand which he believes to be Hyun-seo and starts to flee. When he realizes he isn't holding Hyun-seo, he turns to see the creature running towards her. The creature then snatches Hyun-seo and dives back into the river.During a mass funeral for the victims of the creature (including Hyun-seo, believed dead by her family) Government representatives arrive and forcefully quarantine all those who have been in direct contact with the creature, including Gang-du and his family. The South Korean government announces that the creature is not only a direct danger, but also the host of a deadly, unknown virus (which is the formaldehyde from the drain) as the American (who lost an arm to the creature) is discovered to be infected with it.While in quarantine inside a hospital, Gang-du receives a phone call from Hyun-seo, who is not dead, but trapped somewhere in the sewers by the creature. She is cut off as her phone battery runs out. Gang-du tries to explain this to others, but his protests go ignored by all except his family. All four of them then decide to escape the hospital to track down Hyun-seo. Hee-bong also buys weapons and a map of the sewers from a group of men, so they are prepared for searching for Hyun-seo.Meanwhile two brothers, Se-jin and Se-joo, are searching for food when they are attacked by the creature. They attempt to flee but are both 'eaten' by the creature. The creature then takes both brothers back to the sewers where it regurgitates both of them with only Se-Joo coming out alive. Hyun-seo sees that Se-Joo is alive and they both hide out of sight from the creature.While Gang-Du and his family are searching the sewers and streets for Hyun-Seo they stop at a food stand to rest. The next morning Hee-bong tells Nam-il and Nam-joo that Gang-du can't help being the way he is because of his miserable childhood, which he hypothesizes affected Gang-du's physical health and mentally disabled him. Soon after they are attacked by the creature. They fire at the creature but quickly run out of ammunition. Gang-du, believing he has shells left in his shotgun, gives his gun to his father, at his request, so that he can attempt to hold off the creature, buying his family some time. As Hee-bong is about to fire at the creature, Gang-du realizes he has miscounted his shots, and that his shotgun is empty. As he looks back to his father, Hee-bong is attacked and killed by the creature. Gang-du runs back to his father but the Army and scientists arrive and capture Gang-du. Nam-il and Nam-joo are able to hide but are separated from each other while Gang-Du is taken away, screaming in agony over his father's death.While Nam-joo is searching through the sewers, she comes across the creature and tries to attack it with her bow. She hesitates to take a shot and the creature knocks her down into a hole, knocking her unconscious. The creature unable to reach her continues running through the sewers.After being placed in a special medical facility, Gang-du is interrogated by two scientists. Interpreting his ravings about his still-alive daughter as a product of infection by the virus, they decide to operate on his brain in an effort to extract a samples procedure they deem necessary as until now they have found no actual trace of any virus. As he is being prepared for the operation, however, Gang-du manages to break free. Taking one of the nurses hostage with a syringe of his supposedly-infected blood, he escapes to continue his search for his daughter.Nam-il attempts to get help from a friend who tells him that the Government have placed a bounty on his and his families heads. While in an office building Nam-il is betrayed by his friend for the bounty. Nam-il evades capture from both his friend and the police and flees. Nam-il wakes up the next morning with a homeless man (Yoon Je-moon) sitting next to him. After talking they both join together to try and find Hyun-seo and kill the creature.Back in the sewers, while the creature is sleeping, Hyun-seo ties together clothes from the bodies of the dead to create a rope. She successfully latches the makeshift rope to the top of the drain but while she attempts to jump up to the rope she is caught by the creature. The creature, seemingly still sleeping, gently puts Hyun-seo back on the ground. Terrified, Hyun-seo slowly attempts to hide but the creature leaps at her and Se-joo and swallows the both of them.Meanwhile, the government announce they will release a harmful chemical called Agent Yellow, into the river and its surrounding areas, that they hope will kill the creature. People against Agent Yellow arrive starting to protest its release.Gang-du finally finds the sewers where the creature kept Hyun-seo but the creature and Hyun-seo are not there. As Gang-du climbs down into the sewer the monster passes over him. He sees Hyun-seo's arm hanging out of the creatures mouth and starts to chase after it. As Gang-du is running after the creature he comes across Nam-joo and noticing that it is heading towards the protest, go after it together. As Nam-il and the homeless man arrive at the protest, and as the creature attacks, Agent Yellow is released. Agent Yellow appears to hurt the creature but doesn't kill it. As the creature is laying on the ground, Gang-du sees Hyun-seo in its mouth. He pulls her and Se-joo out but Hyun-seo is revealed to have died clutching Se-joo, who is still alive. In a fit of rage, Gang-du attacks the creature with a metal pole but is knocked to the ground. Nam-il and the homeless man then come to Gang-du's aid, with Nam-il throwing Molotov cocktails at the creature. The homeless man climbs up to the platform above the creature and starts to pour petrol onto it but as Nam-il goes to throw his last bottle he drops it. Nam-joo then picks up the flaming cloth from the bottle with one of her arrows and fires it at the creature, hitting it in its eye. The creature bursts into flames and attempts to run to the Han River when Gang-du appears from behind one of the bridges pillars, with the metal pole in hand, and impales the creature in its mouth, finally killing it.As Nam-il and Nam-joo hold Hyun-seo, mourning her death, Gang-du walks over to Se-joo and picks him up, taking him away from the protest and to safety.Time passes and Gang-du is living with Se-joo in his food stand. At night, Gang-du believes he sees something in the distance, picking up a shotgun, but then realizes it was nothing. Gang-du and Se-joo then sit down to have something to eat, while a news broadcast about the monster attack on the TV is on. The film ends with Se-joo asking Gang-du to turn the TV off so they can concentrate on eating.