- Directors: Bong Joon ho
- Producers: Choi Yong Bae
- Writers: Baek Chul hyun, Bong Joon ho
- Genres: Comedy, Drama, Fantasy, Horror, Sci-Fi, Thriller
- Actors: Song Kang ho, Byeon Hee bong, Park Hae il, Bae Doona, Ko Ah seong
The film opens with an American military pathologist commanding a reluctant Korean assistant to violate protocol by dumping over 200 bottles of formaldehyde down the drain, which leads to the Han River. A few years later, two men are standing and fishing in Han River when one discovers a mutant amphibian (which is never shown). He releases the palm-sized creature when it bites him. Four more years later, a man commits suicide by jumping off a bridge into Han River, but not before noticing that there’s “something dark, underwater.”
In the present day, Park Gang-du (Song Kang-ho) is a seemingly slow-witted man who runs a snack-bar with his father, Hee-bong (Byeon Hee-bong). Hyun-seo (Ko Ah-seong) is a schoolgirl and Gang-du’s daughter. Gang-du’s sister, Nam-joo (Bae Doona), is a national medalist archer who has an unfortunate tendency to hesitate, and his brother Nam-il (Park Hae-il) is an alcoholic former activist who has not done much since graduating from university.
Gang-du is delivering a meal to some patrons, and sees a crowd gathering along Han River. He joins them as they stand near the side of the river and point at something under the Wonhyo Bridge. It is the creature, now grown. It drops into the water, and moves towards shore. Gang-du throws a can of beer into the water near it, and the creature grabs for the can. The other people nearby then begin to playfully toss other pieces of food to it, but the creature disappears from view. A few moments later, the creature appears on shore behind them, and begins to attack and devour people. Gang-du and an American man named Donald try to fight the creature, and successfully hit it with a street sign, Donald however, loses his arm to the creature afterwards. As Gang-du runs away, he sees Hyun-seo emerge from the snack bar and grabs her hand without stopping. He then stumbles and unwittingly grabs a different girl. A short distance away, he looks back and sees the creature pull Hyun-seo into the river. Gang-du then sees the monster dragging her on the opposite bank before disappearing into the water.
As Nam-joo and Nam-il mourn over their dead niece, Gang-du manages to revive Se-ju. In the epilogue, we see Gang-du and Se-ju living as a family in the rebuilt and cozy-looking snack bar, sometime in the winter. One night Gang-du believed he saw something move outside. He gets his rifle but then sets it down, believing it was his imagination. A televised US Senate press release – claiming that the Korean “disease crisis” was caused by “misinformation” – is drowned out by their conversation. The child asks him to turn it off, as he finds it boring, and they eat dinner.