At first it comes on like a grim version of “Sixteen Candles”: a young, flame-haired woman flees her house after being upstaged at her own birthday party (where her older sister makes a happy announcement, with perfect malicious timing), then gets tipsy at a club and ends up with a dodgy boy who turns out to be a creep.

Life is almost comically frustrating for Moll (Jessie Buckley), but “Beast” is no John Hughes scenario.

Moll’s not a teenager anymore, and her stunted existence — she lives with her parents and helps tend to a father with dementia — is shadowed by a troubling incident from her past.

“Beast,” which played in SIFF’s first week, is Michael Pearce’s feature writing-directing debut. The beast stalking the Isle of Jersey — that small enclave of Englishness just off the coast of France — has already killed a handful of people, including a victim slain the night of Moll’s birthday.

Pearce rolls out the story as a whodunit, scattering a few viable suspects around — but Moll’s family, and the police, think the main candidate is Pascal Renouf (Johnny Flynn), the rough, scar-faced young man who came to Moll’s rescue the night she ran away. Moll and Pascal, both cast out by society, rush toward each other as though magnetized.

She knows he could be the killer, but after having been surrounded by dullards on a small island all her life, the intoxication of their chemistry overwhelms her. When an insinuating police officer (Trystan Gravelle) interrogates Moll and asks whether her sex life with Pascal has been out of the ordinary, she contemptuously replies, “It’s not ordinary. It’s amazing.”

This is mad love, always rich turf for the movies. We see Moll taking dangerous risks on Pascal’s account, and we worry about her, but we also sense her exhilaration.

The premise is a little like Nicholas Ray’s great film noir “In a Lonely Place” (1950), where we watch Humphrey Bogart begin a romance with Gloria Grahame while he’s under suspicion for murder — except that “Beast” shows us the dynamic from the female perspective. Pearce adds a sinister undercurrent: Moll, after all, must herself be considered one of the suspects.

I wish “Beast” fulfilled all its early promise, but it stumbles toward the end, and its caricature of domestic asphyxiation seems a little canned — did Moll’s mother (ably played by Geraldine James) have to be quite such a brittle harridan?

The movie is memorable, though, because of the two lead performances. The Irish-born Buckley has seen success in longform TV shows like “Taboo” and “War & Peace;” Flynn is a musician and actor, perhaps best known as the youthful version of Albert Einstein in “Genius.” They’re mesmerizing.

When movie stars are cast as misfits, it can produce unconvincing results (see Michelle Pfeiffer in “Frankie and Johnny” for further information). No such problem here.

Buckley and Flynn are both arresting, and it’ll be surprising if their careers don’t take off, but they don’t come across like stars. They look as though they’d stepped out of the pages of an old folk tale hatched from an insular island culture like Jersey’s: Two phantom spirits, not entirely to be trusted.

‘Beast’ (3 stars)

On the isolated Isle of Jersey, two misfits meet and fall for each other — a situation complicated by the fact that either of them might be a local murderer. This film stumbles toward the end, but the mesmerizing performances by future stars Jessie Buckley and Johnny Flynn make it memorable.

Rating: R, for violence, language

Opening: Meridian, Seattle 10