Running a rubber-gloved finger across gallons of dust-covered bottles of formaldehyde, a US military official (Scott Wilson, setting a darkly humorous tone) orders a Korean morgue attendant at a US Army base in Seoul to “empty every bottle to the very last drop,” despite protests that the toxic chemicals will drain directly into the Han River. This actually happened in 2000. The consequences are not yet known, but in Bong Joon-ho’s The Host, the follow-up to his superlative 2003 thriller Memories of Murder, the spill spawns a genetic mutation — part salamander, part fish, part . . . vagina — that emerges from the Han’s shores six years later. A gloriously realized rampage leaves scores of day-trippers maimed or worse before the scampering 15-foot-long beast drags a young schoolgirl (Ko Ah-sung) into the murky depths. Government officials presume her dead, but not the Parks; her comically dysfunctional family (including Korean stars Song Gang-ho and Bae Du-na) would sacrifice their lives to get her back. It may be a “monster movie,” but a multi-layered satire lurks just beneath the surface as Bong plumbs all manner of pathos and drama. Digs at US foreign policy add a welcome splash of muckraking.