We are deep in “Fatal Attraction” territory with “Greta,” but Neil Jordan’s thriller isn’t about a woman’s fixation on a man, but the destructive obsession of the title character (Isabelle Huppert), an older woman who stalks a younger one, Frances McCullen (Chloe Grace Moretz), for reasons that are only gradually revealed. The pulpishly implausible plot strains one’s ability to suspend disbelief, and the last act is protracted almost beyond endurance, but Huppert’s no-holds-barred performance makes it loopy fun.

The setting is New York City, where Frances, a naïve college grad from out of town, lives with her best friend Erica Penn (Maika Monroe) in one of those gorgeous apartments movie characters frequently inhabit in the Big Apple but the rest of us could never afford. Riding home one night from her job as a waitress in a tony restaurant, Frances finds a lost handbag and, despite Erica’s misgivings, decides to return it personally.

The owner is Greta Hideg (Huppert), who lives in an old-fashioned flat in the interior of a red-brick courtyard, and after thanking the young woman profusely, she graciously invites the young woman in for tea. Frances finds the elegant Greta, who speaks with a French accent and plays the piano, rather endearing—even when she has to shush the noisy neighbors, whose renovation project involves loud pounding on the walls.

Erica, of course, warns her roommate not to get too close, but Frances develops a friendship with Greta, whose deep loneliness apparently stems from the absence of her daughter, who she explains is off studying in Paris. Friendship, however, soon turns into a menacing dependence, as Greta grows increasingly clingy and demanding of the young woman’s time and attention. When Frances accidentally learns that there’s more to the lost purse than she originally assumed and tries to put some distance between them, Greta snaps, menacingly following both her and Erica and causing a scene at the restaurant so venomous that staff and police have to cart her off for observation.

But she’s soon out again, and the authorities can do little to stop her stalking.

It would be unfair to reveal too much about what follows, except to say that Greta is not a person to give up easily and Frances is entirely oblivious to Erica’s warnings that no good deed goes unpunished. Add to this the fact that Huppert plays unhinged with absolute abandon (at one point even lurching into a nutty dance of malignant joy), and Frances is eventually compelled to suffer wild indignities as the cat-and-mouse game between the women reaches outlandish proportions, with knives and syringes entering the picture at appropriate moments. The gore reaches Grand Guignol proportions, though none of it is realistic enough to disturb the air of a dark, fractured fairy tale.

Screenwriter Ray Wright does prolong things astronomically in the final act, adding a story thread involving a private detective (Stephen Rea) hired by Frances’ father (Colm Feore) that in the end provides no more closure than Scatman Crothers’ one did in “The Shining.” (Rea, of course, is a sort of lucky charm for Jordan, so there must have been special inducement to squeeze in a role for him.) And a tricky final twist may strike many viewers as a bridge too far, though it allows for a snarky conclusion in keeping with the movie’s sense of humor.

While “Greta” is basically B-movie goofiness, however, it’s served up with zest, thanks especially to Jordan, working in what is basically early De Palma mode (though without the extreme flourishes—Anna Rachard’s production design and Seamus McGarvey’s cinematography are sleek without being overbearing). Moretz goes along gamely with the nonsense, and Monroe is especially sharp as a blonde who turns out to be much less ditzy than one might assume at first.

At one point in the film Greta compares Frances to a bottle of wine that “promises a lot but disappoints.” “Greta,” on the other hand, doesn’t promise all that much, but it delivers.