The rhythms of life for meek Hye-ja (Kim Hye-ja) are simple: chopping leaves for the herbal medicines she sells in her little shop, making tea in the afternoons, worrying about her mildly retarded adult son, Do-joon (Bin Won), and extricating him from whatever minor disaster he’s got himself into this time. The horror of South Korean filmmaker Bong Joon-ho’s (The Host) bleakly comic film begins with how it’s barely a blip in her routine when Do-joon is accused of murdering a teenaged schoolgirl and Hye-ja must doggedly go to work to find the real killer (the police being hapless and hopeless in this regard). Bong dredges up dread from the mousy determination of Hye-ja’s Miss Marple-like detective work — her meekness hides an astonishing fortitude — and from the increasingly desperate and extreme actions she takes in the course of it. The blackest of humor comes in one question that hovers over the film for a long while — What if Hye-ja discovers that Do-joon is actually guilty of the crime he’s accused of? — and from the other question that, while previously asked by cinema — How far will a mother go to protect her child? — has rarely gotten this profoundly disturbing an answer.