NOTE: This spoiler was sent in by Altressa who calls the film... "Very entertaining.”
The movie begins in the year 2000 in the morgue of a US Army base in Korea; an American doctor is standing next to a row of medical cabinets. He comments to the other man in the room how dusty it is and how he hates dust. The other man (possibly his assistant who is Korean) tells the doctor he will clean again. The doctor tells him that it won’t be necessary and instead tells the assistant to dump the several dozen bottles of Formaldehyde down the drain. The assistant protests and tells the doctor that it will drain into the Han River and the chemicals are highly toxic and need to be properly disposed of and not down a sink. The doctor then makes it an order and in the next scene the assistant is emptying a bottle into the sink and is wearing a gas mask as a huge cloud of smoke continuously engulfs him. The camera slowly drifts to the table behind him and pans across dozens upon dozens of emptied bottles.
The next scene begins in the year 2002 with two men fishing in the Han River. One of the men sees something small swim by him. He then uses a small pink cup he is holding to dip in the water and catch it. They comment on how many tails it has and how they have never seen anything like it before. The river ebbs and the man holding the cup drops it, losing the creature.
Cut to the year 2006 and a man is standing on a small ledge of a bridge about to commit suicide. The man hanging on the ledge whispers to himself about something in the water. Below is a huge swirl and a dark undistinguishable figure is circling. The man then plummets to his death.
Next scene, a man, Gang-du, is asleep and drooling with his head resting on packaged food products. As the camera pulls back we see he is in a small convenience store trailer filled with gum, cigarettes, snack foods, etc. We then realize the small store is located on the bank of the Han River and catering to beach-goers.
His daughter Hyun-seo arrives home from school. She’s upset for her father not being able to come to some school event and he makes excuses as to why he couldn’t come and she quickly changes the subject to a sporting event about to come on TV and how they need to hurry to go watch it. The father and daughter turn on the TV in time to catch a news special about the body of a man who committed suicide by jumping off a bridge, was missing his lower half. They quickly change the channel to watch the girls Aunt in an archery competition who ends up taking the bronze medal.
The girl's father Gang-du, is scolded by his father for eating a customer’s food (a grilled squid leg). To apologize, his father makes him take the customers free beer. When he arrives he sees people have gathered at the edge of the cement dam looking at a huge creature curled into a ball and suspended by its tail underneath a bridge. The creature drops into the water and swims closer to the people and begins to circle in front of them. The crowd then begins throwing things at it and the creature swims off. Everyone returns to their mats and just as Gang-du is about to give the remaining beer to the people on the mat he hears screams as the creature is now running down the bank of the dam. Everyone scatters and chaos ensues. The creature chases people all through the beach and some even get pinned in a small trailer petting zoo that is closed and chained at the opposite end. An American and Gang-Du try to force the door open but it is torn open by the massive amount of escaping bodies. The American and Gang-Du pursue the creature to stop it. A slower man is swallowed whole and the American’s arm is pinned by the creature. During the fight Gang-Du injures the creature’s tail with a pole from a street sign and its blood sprays him in the face.
Meanwhile at the food stand, Hyun-seo emerges from the trailer and sees people running. Her father snatches her hand and runs her down the beach. They trip and he grabs a hand and runs again, only realizing that it is not his daughter but a stranger’s child. He looks back and his daughter is snatched up by the tail of the creature and dragged to the opposite beach. In the distance we see the creature regurgitate a body and then swallow another body just before it dives back into the water.
The father goes back to the trailer and the authorities are forcing people to evacuate. The evacuees are all confined in what looks like a school gym and pictures of victims line the room. The girl’s grandfather, father and uncle are on the floor with dozens of others who are all mourning. The girls’ aunt enters and approaches the picture of the girl, while holding her bronze medal in her hand. The Aunt falls onto the picture of the girl and then the whole family falls to the ground and begins to wriggle and moan and cry loudly. So loudly it drowns out the mourning cries of the all the others and news photographers begin to take pictures of the entire family rolling around on the ground.
A man in a yellow decontamination suit enters and asks if anyone had come in contact with the creature. The family pulls itself together and sits crying yet peacefully next to each other. The man in the yellow suits tells everyone that there is no reason to panic, and asks for people to just volunteer information about their contact with the creature. The girls’ father confesses that the creature’s blood hit his face and he is forcibly detained and taken to a local hospital along with the rest of his family.
The news reports that the creature will spread a deadly virus to anyone who has come in contact with it and that the American soldier who was attacked earlier survived but has lost an arm.
Gang-Du has his cell phone on the hospital stand next to him, it rings and it’s Hyun-seo with a bad connection, trying to tell him she is in a sewer. The call ends and the family wakes to see their brother saying his daughter’s name. The family believes she is alive but when they tell a police officer in the hospital he doesn’t believe them. They devise an escape plan, which is comically carried out.
Their father had arranged for some undesirable men to aid them and pays them over $11,000 for a sewer map, guns and a stolen truck. The children, who are still in the getaway vehicle, watch themselves on TV as wanted fugitives for spreading the virus. They all get into the new stolen truck and head back down to the beach, which has now been quarantined. They disguise themselves and enter as a decontamination team.
Their sewer adventure, which includes a family member falling out of a tunnel to shooting dripping water, ends with not finding the girl. Meanwhile the girl is in a sewer room with just high walls and a small dead end tunnel. Only half a grate hangs above her. The bodies of the people that were taken from the beach, the same day she was abducted, surround her. She is the only one alive.
The family heads to their shop on the beach and the next morning they see the creature a few yards from their trailer with its mouth open to the rain. The father shoots the creature and it attacks them, rolling the trailer over. They get out and the girl’s uncle shoots it and it runs off and everyone give chase until they run out of ammo. The creature escapes under a bridge and hangs from its tail, drops into the water and swims towards them. Gang-Du tells his father he has one shot left in his gun and gives it to him. The father tells his children to leave and find Hyun-seo. They all run ahead and the father turns, takes aim and fires nothing. Gang-Du realizes he had miscounted his shots and stops to help his father but it’s too late. The creature charges the grandfather, killing him. Then it turns and snatches him by the leg and slams his body into the ground just before fleeing into the river. Gang-du stays with his father’s body as his siblings are forced to flee since the authorities heard the gunfire and were closing in on them.
(A few of the next events may be a bit out of sequence since they were supposed to be happening simultaneously):
The uncle contacts a former colleague and asks him to pinpoint the cell phone call that Hyun-seo made. The man helps him but betrays him to an angry mob that thinks he’s got the virus. (Somewhere in here we learn that a biological chemical known as Agent Yellow, is to be released to kill the creature and that the city must evacuate) He escapes from the mob and is injured when he falls from a bridge. Just before losing consciousness, he is able to call Gang-Du’s cell phone and tells him where Hyun-seo is and then text messages his sister the sewer name also.
While Gang-Du is in custody an American doctor who doesn’t know that Gang-Du understands English, reveals that there was no virus. Unfortunately for Gang-Du the doctor doesn’t want to tell the world that there is no virus so he wants to take a sample of his brain tissue.
The aunt makes her way back to the small shop and charges up her cell phone and sees the text message. She heads to the sewer with her archery gear while Gang-Du undergoes brain tissue extraction. She confronts the creature who charges her and knocks her out cold and into an adjacent pit.
The uncle has now regained consciousness finds himself under a bridge and next to a homeless man. The uncle wakes in a panic and asks the homeless man to tell him where a particular sewer is. The Uncle begins collecting bottles and gives him his wallet for payment. The homeless man leads the uncle to the sewer.
The entire time Hyun-seo who is still in the sewer had been watching the creature drop people’s bodies into the room and most of the people who originally started out in there with her were all but eaten. A little boy who had been brought in, a few bodies after her arrival, and survived had been hiding in the small tunnel with her. Hyun-seo had devised a plan of escape by taking the dead people’s clothing and making a rope.
Gang-Du who is now in a vegetable like state sits in a hospital room. He is of course faking and attacks a nurse who has just taken his blood. He grabs the syringe from her and threatens to infect everyone with his blood if they don’t get out of his way. Gang-du escapes and heads towards the sewer to find Hyan-seo.
So now everyone is either at the sewer or headed there and Hyan-seo springs her escape plan into action. But the creature who decides to sleep in their only path of escape foils her plan. She tries again but the creature swallows her and the boy.
Meanwhile protesters to the release of agent yellow have gathered on the edge of the beaches dam. The aunt awakens in time to see the creature speeding past her in to the river. The creature surfaces and sees all the humans who have collected in one spot and heads for them. The aunt follows.
The uncle had been making Molotov cocktails in a cab ride to the protest. Over the protesters heads hangs a huge silver ball and is leaking a yellow gas.
The creature emerges from the water and begins to wreak havoc on the crowd. The silver ball drops a humungous cloud of yellow gas falls like sand over the creature. The creature flops on its back and protesters who weren’t fast enough to escape were enveloped by the cloud and pass out.
The girl’s father arrives and approaches the creature and sees a hand in the mouth and pulls out to reveal his daughter tightly clinging to the boy. The uncle arrives in time to toss flaming cocktails at the creature, which just won’t die, but misses all of them and even drops one.
The homeless man, from overhead, pours fuel all over the creature and the Aunt arrives in time to use the lit rag from the broken bottle to adorn the tip of her arrow. She fires and the creature is consumed in flames. It frantically tries to escape back to the river but Gang-du stabs it and it falls dead.
The family is crying and gathered around the body of Hyun-seo. Gang-du cries over the body of the boy and his eyes slowly open while his daughter’s never do.
Later we see Gang-du and the boy eating dinner watching TV in the little shop on the beach. The TV reports on the virus, (from some time ago, not sure how long) in relation to the creature, and how the virus did not exist.