The film revolves around Park Hee-bong, a man in his late 60s. Park runs a small snack bar on the banks of the Han River and lives with his two sons, one daughter, and one granddaughter. The Parks seem to lead a quite ordinary and peaceful life, but maybe a bit poorer than the average Seoulite. Hee-bong's elder son Gang-du is an immature and incompetent man in his 40s, whose wife left home long ago. Nam-il is the youngest son, an unemployed grumbler, and daughter Nam-joo is an archery medalist and member of the national team. One day, an unidentified monster suddenly appears from the depths of the Han River and spreads panic and death, and Gang-du's daughter Hyun-seo is carried off by the monster and disappears. All the family members are in a great agony because they lost someone very dear to them. But when they find out she is still alive, they resolve to save her.

The event described in the beginning of the film is based on an actual event. In February 2000 at a US military facility located in the center of Seoul, a US military civilian employee named Mr. McFarland was ordered to dispose of formaldehyde by dumping it into the sewer system that led to the Han River, despite the objection of a South Korean subordinate. The government attempted to prosecute Mr. McFarland in court, but the US military refused to hand over the custody of Mr. McFarland to the South Korean legal system. Later, a South Korean judge convicted Mr. McFarland in absentia. The Public was enraged at the government's inability to enforce its law on its own soil. In 2005, nearly five years after the original incident, Mr. McFarland was finally found guilty in a court in his presence. However, he never served the actual prison sentence, and there have been no sightings of a mutant creature in the Han River--yet.

US Doctor in Morgue:
Mr. Kim,

[running a finger along the counter]


US Doctor in Morgue:
I hate dust more than anything.


Young Korean Doctor:
Oh, I will clean again.


US Doctor in Morgue:
You don't have to clean up now. Why don't you dump this first?


Young Korean Doctor:
That's formaline.


US Doctor in Morgue:
Formaldehyde, to be precise. To be even more precise, dirty formaldehyde. Every bottle is coated with layers of dust. Pour 'em into the sink.


Young Korean Doctor:
Excuse me?


US Doctor in Morgue:
Just empty every bottle to the very last drop.


Young Korean Doctor:
It's just - They are toxic chemicals, and the regulations state -


US Doctor in Morgue:
Pour them right down the drain, Mr. Kim.


Young Korean Doctor:
If I pour them in the drain, they'll run into the Han River.


US Doctor in Morgue:
That's right. Let's just dump them in the Han River.


Young Korean Doctor:
But, you know, this is not just any toxic chemicals -


US Doctor in Morgue:
The Han River is very broad, Mr. Kim. Let's try to be broad-minded about this. Anyway, that's an order. So, start pouring.

Revealing mistakes: When Kang-ho Song's character receives a tissue sample, there is no wound in the remaining scenes, even though he was screaming in pain.