Depth From Above: The Host uses a mutant monster to make a political satire.

While most of today’s better-known Korean directors are specialists—Oldboy director Park Chan-wook, for example, does revenge and only revenge—Bong Joon-ho has yet to settle into a stylistic groove. He followed 2000’s Barking Dogs Never Bite, a dark comedy, with 2003’s Memories of Murder, a naturalistic crime drama. Now, lumbering from the polluted Han River, comes The Host, the tale of a fish who suffers chemical poisoning and grows not ill, but big and bad. A mere four years after an imperious American military doctor orders a Korean assistant to pour large quantities of formaldehyde down the drain and thus into the Han, the mutated creature attacks the people who frequent Seoul’s riverfront. The ensuing panic is as big as, well, Godzilla.

Following precedent, Bong and co-scripter Baek Chul-hyun call in the army, supplemented by the American troops whose lengthy presence in Korea is never far from The Host’s satirical agenda. But the essential battle falls to an unusual family whose campaign against the overgrown fish represents the redemption of low-status Koreans. When plucky, motherless schoolgirl Hyun-seo (Ko A-sung) is kidnapped by the beast—who tends just to gobble his other victims—her family finally gets itself together. Dad is sleepy slacker Gang-du (Song Kang-ho), who helps run a junk-food stall along the river, aunt Nam-joo (Bae Doo-na) is a skilled archer who always chokes in the heat of competition, and uncle Nam-il (Park Hae-il) is a college graduate and former student revolutionary who turned to the bottle after failing to get a job. Add a homeless boy who becomes Hyun-seo’s fellow prisoner, and the creature (called “the host” because of a virus subplot and possible AIDS allegory that goes nowhere) faces a formidable army of societal losers.

The film’s political commentary is far from subtle, but it’s offered with winning good humor. (At last Nam-il has an acceptable target for the Molotov cocktails he used to wield against the government.) When he’s not trying to be funny, Bong stages a few momentarily effective shocks and adds to the general creepiness by showing some alarming medical procedures. But the plot meanders, and the monster—though rendered in state-of-the-art CGI—seldom seems to be in the same movie as the human characters. Bong really hasn’t settled on a genre this time, and the outcome is a fitfully amusing disappointment.