The same mixture of intensity and bleak comedy that Korean director Bong Joon-ho brought to giant monster movies in “The Host” he now applies to the suspense genre in “Mother,” and the result is a typically strange but very satisfying twist on Hitchcock’s “Psycho.” And like that classic, it’s both creepy and fun.

The title figure is played by Kim Kye-ka, and runs a small herb shop in an unnamed Korean town. She’s utterly devoted to her 27-year old son Do-jun (Weon Bin), pampering him with specially-prepared meals and even sleeping with him (platonically, of course). But the young man’s diminished mental capacity makes him a willing dupe to his childhood pal Jin-tae (Jin Gu), a slick hood whose propensity for getting the boy into trouble leads Mom to consider him a bad influence. Her suspicions are confirmed when, after Do-jun is swiped by a car, Jin-tae and Do-jun track the auto to a golf course and attack the driver and his party, landing them in trouble with the cops.

Do-jun’s real problem, however, occurs late one night when, after getting drunk waiting for Jin-tae, he encounters a schoolgirl walking home and follows her. When the girl’s found murdered and a golf ball on which Do-jun’s scrawled his name is found beside the body, he’s arrested and presumed guilty by the indolent, ineffectual police.

Naturally his mother is sure her son is innocent, and after an unhappy experience with a sleazy lawyer who suggests a plea bargain, undertakes an investigation of her own. She begins with the belief that Jin-tae is the real culprit, and sneaks into his house (where she spies on him while he’s having sex with another schoolgirl). But that’s only the beginning of a search that takes her to the dead girl’s funeral—where her presence causes a scene—and to students who engage in kinky sex, which they also photograph, and to an old peddler who might have witnessed the crime—or committed it. In trying to prove Do-jun’s innocence his mother will go to positively frightening lengths, and in the process part of the reason behind her devotion is revealed.

If you look at “Mother” from the “Psycho” perspective, what you find is a reversal in which it’s not the son who obsessively protects the false memory of his mother, but the mother who harbors an unrealistic idea of her son and will do virtually anything to defend him. And it contains a similar strain of dark humor to accompany the sudden bursts of violence, as well as a twist ending that can’t match Hitchcock’s but provides a satisfyingly perverted close.

The script, which Bong co-wrote, and the director’s penchant for creating an odd, sinister mood, abetted by Hong Gyeong-pyo’s atmospheric cinematography and Lee Byeong-woo’s supportive score, are essential to the picture’s success. But they would count for little without the cast. Jin is genuinely threatening as Jin-tae, and Weon brings a surprising degree of depth to the troubled son. But it’s Kim who anchors the film with her stunning performance as a woman whose seemingly mild manner masks an absolutely implacable will. By turns mousy and furiously vengeful, she creates a portrait of motherhood far from the sentimental one so often encountered in films, especially those of eastern provenance.

At the beginning of “Mother,” Kim is photographed doing a weird dance in a field. The same image reappears at the close. It somehow captures the peculiarly fascinating tone of a film that turns genre conventions on their head in creating a thoroughly engrossing mixture of murder mystery, psychological thriller and very black comedy.