The titular animal of Bong Joon-ho’s “Okja” is so endearing that she even defecates cutely, depositing little fecal pellets into a stream in a way that somehow connotes synchronicity with nature.

This fictional creature – a giant, genetically engineered pig created to provide a cheap corporate superfood – is a major achievement on the part of Bong and his collaborators, suggesting a balletic hippopotamus with the ears and personality of a Labrador retriever.

Like many otherworldly martyrs in metaphorical genre films, Okja is a gentle giant, and the contrast between her bigness, potential ferocity, vulnerability and eagerness to be loved by humans is unforgettably poignant.

For a while, Okja is lucky. She has someone who honors that love: the almost adolescent Korean farm girl, Mija (Ahn Seo-Hyun). The first half of “Okja” is the most surpassingly lovely passage in Bong’s career, abounding in the casual magic that one associates with a Hayao Miyazaki or an early Steven Spielberg film.

Okja is first seen emerging from the trees, which Bong stages with a lack of ceremony that connotes deep respect — he doesn’t give her a forbidding, portentous movie-monster entrance. Instead, Okja lumbers into the frame, joining Mija in the wilderness where the girl lives with her grandfather (Byun Hee-bong) who believes that it’s time for Mija to outgrow her childhood pet.

These moments, which Bong stages with lucid rapture, serve a clear purpose: We don’t want them to end, even though we know the filmmaker’s softening us for the kill. One is never simply allowed to enjoy the companionship between children and their magical friends in these sorts of fantasies. Their relationship must be imperiled in the service of making a point about the inherent corruption of human society.

A prologue, set in 2007, provides a hint as to where this film is headed. In New York, Lucy (Tilda Swinton), the head of the Mirando Corp., announces to the press that a new pig has been discovered that will lead a revolution in solving world hunger.

For publicity reasons that make no sense even by the irrational standards of satire, several of these pigs are dispersed to farmers across the world for a contest. After 10 years, the biggest pig will be celebrated and its farmer rewarded. Guess which pig is the biggest, and who’s heading toward a corporate guillotine?

The Mirando Corp. takes Okja out of the Korean wilderness and into New York, stuffing her into an undersized truck that suggests the notoriously awful way calves are crammed into small cages to tenderize them for veal cutlets. At its halfway point, the film’s tone dramatically changes, shifting from a children’s fantasy into a broad and startlingly cruel parody.

Imagine if E.T. were kidnapped, taken to a slaughterhouse, raped and generally worked over for much of that film’s second act, and you’ve got an idea of the emotional discombobulation that drives “Okja”. Bong betrays our trust to show how we betray animals, by torturing and slaughtering them for food that we gobble up without question.

Bong, though, can’t leave well enough alone. The awfulness of Okja’s violation, and the operatic horror of the climactic images of a slaughterhouse that’s killing thousands of these pigs, speak for themselves, but Bong often stops the second half of the film in its tracks for sophomoric caricature that’s below his talents.

Lucy is another of Swinton’s gratingly mannered freak-show creations, in which two-thirds of the acting is seemingly accomplished through costume changes. And Jake Gyllenhaal — not one for subtlety lately anyway — indulges in the most ridiculous showboating of his career, playing an animal-rights charlatan who suggests a blend of Steve Irwin and Geraldo Rivera’s modes of zealotry.

These performances inadvertently allow us to feel superior to the corporations that we lambast in conversation and enable in reality, diluting our complicity in Okja’s suffering — just as we were allowed to feel comfortably, distantly superior to the Haves of Bong’s overrated “Snowpiercer.”

And so the poetry of Okja and of Ahn’s wonderful performance are lost, until a stirring conclusion in frenetic clutter as Swinton and Gyllenhaal chew the scenery against the clinging and clanging of well-staged yet repetitive action set pieces.