‘Mother’

South Korean filmmaker Bong Joon-ho loves to color outside the lines. In “The Host,” his best-known film in the United States, a giant-monster movie doubled as the story of a dysfunctional family (think “Godzilla” by way of “Ordinary People”).

“Mother,” Bong’s fourth film, continues that unpredictable tradition. The movie is built around a tour-de-force performance by Kim Hye-ja, whose monstrously devoted mama takes her place next to her sisters in “Psycho” and “Mommie Dearest,” albeit for entirely different reasons.

Mother (she is never named) sells herbs for a living and practices some illegal acupuncture on the side to help ends meet, but everything she does is for the sake of her 27-year-old son Do-joon (Won Bin), who is a little slow in the head, prone to short-term memory loss and often behaves like a 12-year-old.

Mother watches over her son obsessively: She’s the only person in the world he has — and vice versa (“You and I are one,” she tells him). So when a local schoolgirl is found murdered, and Do-joon is arrested for the crime, Mother jumps into action. She will do just about anything to clear her son’s name, even though the film keeps us guessing as to whether he’s actually guilty. Just when you think you know where “Mother” is going, Bong, as is his wont, pushes the film into a different, much darker direction.

The movie’s utter lack of predictability helps to keep you engaged, even if some of the plot turns are a bit baffling, and the unusual depth and complexity of the characters provide its unusual, scalding power. You’ve never met a mother quite like this one.