The catalyst is an arrogant American director of a lab in South Korea. “I don’t like dust,” he proclaims and orders a Korean assistant to dump down a drain dusty bottles containing formaldehyde. The assistant protests that the drainage will go into the Han River and pollute it. “That’s an order!” he is told and he obeys. Years later, out of the river springs this people-hungry creature, served to us with fabulous special effects. It jumps, scampers on the ground, dives, back-flips and creates mayhem. It deposits victims still alive and disgorged from its massive mouth. There are also rumors a virus is being spread.

The twist in this particular amusement is a family that fights the beast. The unlikely hero is slovenly Gang-du (Song Kang-ho) who works at a food stand. When his daughter, Hyun-seo (Ko A-sung) is captured by the river creature, Gang-du rises to the occasion and sets out on his rescue mission. At first thought dead, the girl is still alive and indulges in heroics of her own.

This is a monster flick with cell phones. If a phone works, a victim not munched to death can call for help. That’s just one of the amusing ploys involved in this film that thrives on mining laughs from mishaps and character flaws even as it builds tension and suspense.